cpu/stm32f0: uart: make sure that 'port' and 'dev' are !=NULL
cpu/stm32f0: uart: make sure that 'port' and 'dev' are !=NULL

This fixes some cppcheck nullPointer errors.

ant9000/RIOT,adjih/RIOT,gebart/RIOT,roberthartung/RIOT,LudwigOrtmann/RIOT,Hyungsin/RIOT-OS,aeneby/RIOT,gautric/RIOT,roberthartung/RIOT,TobiasFredersdorf/RIOT,ant9000/RIOT,Hyungsin/RIOT-OS,OlegHahm/RIOT,jfischer-phytec-iot/RIOT,neumodisch/RIOT,OTAkeys/RIOT,jasonatran/RIOT,yogo1212/RIOT,mtausig/RIOT,dkm/RIOT,neiljay/RIOT,miri64/RIOT,immesys/RiSyn,biboc/RIOT,alignan/RIOT,katezilla/RIOT,plushvoxel/RIOT,josephnoir/RIOT,rfuentess/RIOT,gebart/RIOT,smlng/RIOT,avmelnikoff/RIOT,Ell-i/RIOT,toonst/RIOT,toonst/RIOT,Hyungsin/RIOT-OS,gautric/RIOT,katezilla/RIOT,neiljay/RIOT,thomaseichinger/RIOT,adrianghc/RIOT,Hyungsin/RIOT-OS,kYc0o/RIOT,dkm/RIOT,ks156/RIOT,basilfx/RIOT,kaleb-himes/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,dailab/RIOT,beurdouche/RIOT,RIOT-OS/RIOT,lazytech-org/RIOT,beurdouche/RIOT,dailab/RIOT,tfar/RIOT,aeneby/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,LudwigKnuepfer/RIOT,miri64/RIOT,kaspar030/RIOT,alignan/RIOT,biboc/RIOT,neiljay/RIOT,BytesGalore/RIOT,alignan/RIOT,kaspar030/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,lazytech-org/RIOT,adrianghc/RIOT,backenklee/RIOT,josephnoir/RIOT,josephnoir/RIOT,authmillenon/RIOT,kbumsik/RIOT,roberthartung/RIOT,lazytech-org/RIOT,alignan/RIOT,LudwigOrtmann/RIOT,kb2ma/RIOT,A-Paul/RIOT,immesys/RiSyn,gebart/RIOT,kerneltask/RIOT,kaleb-himes/RIOT,jbeyerstedt/RIOT-OTA-update,kaleb-himes/RIOT,hamilton-mote/RIOT-OS,x3ro/RIOT,Josar/RIOT,mtausig/RIOT,authmillenon/RIOT,cladmi/RIOT,ks156/RIOT,neumodisch/RIOT,rfuentess/RIOT,neumodisch/RIOT,Ell-i/RIOT,jasonatran/RIOT,dailab/RIOT,kYc0o/RIOT,alignan/RIOT,thomaseichinger/RIOT,gebart/RIOT,dkm/RIOT,hamilton-mote/RIOT-OS,neiljay/RIOT,kb2ma/RIOT,kerneltask/RIOT,yogo1212/RIOT,kYc0o/RIOT,RIOT-OS/RIOT,LudwigKnuepfer/RIOT,kb2ma/RIOT,kb2ma/RIOT,basilfx/RIOT,biboc/RIOT,miri64/RIOT,mtausig/RIOT,LudwigOrtmann/RIOT,RIOT-OS/RIOT,kerneltask/RIOT,thomaseichinger/RIOT,gebart/RIOT,beurdouche/RIOT,Josar/RIOT,smlng/RIOT,rfuentess/RIOT,thomaseichinger/RIOT,kb2ma/RIOT,roberthartung/RIOT,avmelnikoff/RIOT,authmillenon/RIOT,syin2/RIOT,basilfx/RIOT,hamilton-mote/RIOT-OS,miri64/RIOT,roberthartung/RIOT,neumodisch/RIOT,hamilton-mote/RIOT-OS,hamilton-mote/RIOT-OS,mfrey/RIOT,LudwigKnuepfer/RIOT,beurdouche/RIOT,jbeyerstedt/RIOT-OTA-update,TobiasFredersdorf/RIOT,jfischer-phytec-iot/RIOT,katezilla/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,backenklee/RIOT,backenklee/RIOT,katezilla/RIOT,Ell-i/RIOT,adjih/RIOT,tfar/RIOT,dailab/RIOT,plushvoxel/RIOT,jfischer-phytec-iot/RIOT,syin2/RIOT,authmillenon/RIOT,plushvoxel/RIOT,lazytech-org/RIOT,basilfx/RIOT,LudwigOrtmann/RIOT,cladmi/RIOT,cladmi/RIOT,Josar/RIOT,adrianghc/RIOT,ant9000/RIOT,kaleb-himes/RIOT,rfuentess/RIOT,toonst/RIOT,A-Paul/RIOT,OlegHahm/RIOT,TobiasFredersdorf/RIOT,cladmi/RIOT,immesys/RiSyn,kYc0o/RIOT,kerneltask/RIOT,kaspar030/RIOT,adjih/RIOT,immesys/RiSyn,Hyungsin/RIOT-OS,miri64/RIOT,avmelnikoff/RIOT,Josar/RIOT,BytesGalore/RIOT,TobiasFredersdorf/RIOT,plushvoxel/RIOT,gautric/RIOT,kbumsik/RIOT,LudwigOrtmann/RIOT,katezilla/RIOT,LudwigOrtmann/RIOT,gautric/RIOT,aeneby/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,dkm/RIOT,jfischer-phytec-iot/RIOT,ks156/RIOT,thomaseichinger/RIOT,LudwigKnuepfer/RIOT,ks156/RIOT,kaspar030/RIOT,OlegHahm/RIOT,BytesGalore/RIOT,smlng/RIOT,kbumsik/RIOT,adrianghc/RIOT,x3ro/RIOT,tfar/RIOT,syin2/RIOT,LudwigKnuepfer/RIOT,cladmi/RIOT,jbeyerstedt/RIOT-OTA-update,ant9000/RIOT,RIOT-OS/RIOT,backenklee/RIOT,ks156/RIOT,OTAkeys/RIOT,basilfx/RIOT,jbeyerstedt/RIOT-OTA-update,kerneltask/RIOT,mtausig/RIOT,mtausig/RIOT,x3ro/RIOT,kaspar030/RIOT,syin2/RIOT,kYc0o/RIOT,syin2/RIOT,dailab/RIOT,OlegHahm/RIOT,avmelnikoff/RIOT,yogo1212/RIOT,smlng/RIOT,beurdouche/RIOT,authmillenon/RIOT,kbumsik/RIOT,TobiasFredersdorf/RIOT,biboc/RIOT,jfischer-phytec-iot/RIOT,tfar/RIOT,ant9000/RIOT,mfrey/RIOT,yogo1212/RIOT,A-Paul/RIOT,neumodisch/RIOT,neiljay/RIOT,gautric/RIOT,toonst/RIOT,Ell-i/RIOT,yogo1212/RIOT,neumodisch/RIOT,kaleb-himes/RIOT,lazytech-org/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,dkm/RIOT,jasonatran/RIOT,jasonatran/RIOT,jbeyerstedt/RIOT-OTA-update,biboc/RIOT,avmelnikoff/RIOT,mfrey/RIOT,OTAkeys/RIOT,adrianghc/RIOT,x3ro/RIOT,kbumsik/RIOT,adjih/RIOT,rfuentess/RIOT,foss-for-synopsys-dwc-arc-processors/RIOT,mfrey/RIOT,josephnoir/RIOT,x3ro/RIOT,immesys/RiSyn,A-Paul/RIOT,plushvoxel/RIOT,adjih/RIOT,Josar/RIOT,BytesGalore/RIOT,jasonatran/RIOT,A-Paul/RIOT,BytesGalore/RIOT,tfar/RIOT,OTAkeys/RIOT,immesys/RiSyn,smlng/RIOT,josephnoir/RIOT,yogo1212/RIOT,mfrey/RIOT,aeneby/RIOT,aeneby/RIOT,Ell-i/RIOT,backenklee/RIOT,toonst/RIOT