Fix crash on warnings when no compiler callback has been set.
Fix crash on warnings when no compiler callback has been set.

beikezcs/yara,frbapolkosnik/yara,smaudet/yara,tdr130/yara,jshufelt/yara,smaudet/yara,jshufelt/yara,Security513/yara,MeteorAdminz/yara,jpypi/yara,VirusTotal/yara,foreni-packages/yara,MeteorAdminz/yara,rayyang2000/yara,jpypi/yara,Security513/yara,jshufelt/yara,plusvic/yara,rayyang2000/yara,VirusTotal/yara,jshufelt/yara,wxsBSD/yara,digideskio/yara,beikezcs/yara,mgoffin/yara,rednaga/yara,hillu/yara,mgoffin/yara,hillu/yara,cblichmann/yara,frbapolkosnik/yara,tanium/yara,kallanreed/yara,kallanreed/yara,smaudet/yara,wxsBSD/yara,pombredanne/yara,VirusTotal/yara,pombredanne/yara,rednaga/yara,plusvic/yara,VirusTotal/yara,hillu/yara,jpypi/yara,kallanreed/yara,neatsun/yara,mgoffin/yara,tanium/yara,cblichmann/yara,hillu/yara,tdr130/yara,neatsun/yara,foreni-packages/yara,bmagistro/yara,Security513/yara,wxsBSD/yara,bmagistro/yara,plusvic/yara,frbapolkosnik/yara,msuvajac/yara,msuvajac/yara,plusvic/yara,digideskio/yara,hillu/yara,plusvic/yara,cblichmann/yara,tdr130/yara,beikezcs/yara,rayyang2000/yara,VirusTotal/yara,msuvajac/yara,mgoffin/yara,rednaga/yara,foreni-packages/yara,pombredanne/yara,neatsun/yara,digideskio/yara,frbapolkosnik/yara,cblichmann/yara,pombredanne/yara,digideskio/yara,wxsBSD/yara,cblichmann/yara,MeteorAdminz/yara,tanium/yara,wxsBSD/yara,pombredanne/yara,bmagistro/yara,smaudet/yara