cpu/esp8266: check free heap before pbuf alloc
cpu/esp8266: check free heap before pbuf alloc

Checking by the send function that at least two maximum size Ethernet frames fit in the remaining heap before the LwIP packet buffer is allocated seems to increase stability. This can be caused by the fact that WLAN hardware interrupts allocate additional memory when receiving a frame during the send attempt.
ant9000/RIOT,toonst/RIOT,mtausig/RIOT,mfrey/RIOT,mtausig/RIOT,mtausig/RIOT,OTAkeys/RIOT,x3ro/RIOT,RIOT-OS/RIOT,A-Paul/RIOT,OTAkeys/RIOT,OTAkeys/RIOT,miri64/RIOT,BytesGalore/RIOT,aeneby/RIOT,aeneby/RIOT,cladmi/RIOT,rfuentess/RIOT,OTAkeys/RIOT,toonst/RIOT,josephnoir/RIOT,x3ro/RIOT,OlegHahm/RIOT,lazytech-org/RIOT,authmillenon/RIOT,josephnoir/RIOT,cladmi/RIOT,BytesGalore/RIOT,kYc0o/RIOT,ant9000/RIOT,smlng/RIOT,smlng/RIOT,A-Paul/RIOT,lazytech-org/RIOT,jasonatran/RIOT,miri64/RIOT,toonst/RIOT,kYc0o/RIOT,OlegHahm/RIOT,jasonatran/RIOT,BytesGalore/RIOT,smlng/RIOT,yogo1212/RIOT,kaspar030/RIOT,lazytech-org/RIOT,mfrey/RIOT,yogo1212/RIOT,x3ro/RIOT,lazytech-org/RIOT,toonst/RIOT,x3ro/RIOT,OlegHahm/RIOT,miri64/RIOT,kaspar030/RIOT,smlng/RIOT,mfrey/RIOT,mtausig/RIOT,ant9000/RIOT,rfuentess/RIOT,miri64/RIOT,kYc0o/RIOT,authmillenon/RIOT,OlegHahm/RIOT,OTAkeys/RIOT,yogo1212/RIOT,rfuentess/RIOT,yogo1212/RIOT,josephnoir/RIOT,authmillenon/RIOT,jasonatran/RIOT,aeneby/RIOT,kaspar030/RIOT,A-Paul/RIOT,authmillenon/RIOT,A-Paul/RIOT,toonst/RIOT,basilfx/RIOT,mfrey/RIOT,kYc0o/RIOT,cladmi/RIOT,aeneby/RIOT,basilfx/RIOT,smlng/RIOT,basilfx/RIOT,authmillenon/RIOT,x3ro/RIOT,josephnoir/RIOT,yogo1212/RIOT,BytesGalore/RIOT,OlegHahm/RIOT,A-Paul/RIOT,RIOT-OS/RIOT,ant9000/RIOT,mfrey/RIOT,RIOT-OS/RIOT,RIOT-OS/RIOT,aeneby/RIOT,kYc0o/RIOT,basilfx/RIOT,BytesGalore/RIOT,yogo1212/RIOT,miri64/RIOT,kaspar030/RIOT,rfuentess/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,ant9000/RIOT,josephnoir/RIOT,jasonatran/RIOT,lazytech-org/RIOT,jasonatran/RIOT,cladmi/RIOT,basilfx/RIOT,rfuentess/RIOT,kaspar030/RIOT,mtausig/RIOT,cladmi/RIOT