Fix issues in stack.c
Fix issues in stack.c

pombredanne/yara,pombredanne/yara,wxsBSD/yara,plusvic/yara,wxsBSD/yara,VirusTotal/yara,pombredanne/yara,VirusTotal/yara,hillu/yara,VirusTotal/yara,VirusTotal/yara,hillu/yara,wxsBSD/yara,pombredanne/yara,wxsBSD/yara,hillu/yara,hillu/yara,pombredanne/yara,wxsBSD/yara,cblichmann/yara,cblichmann/yara,cblichmann/yara,cblichmann/yara,plusvic/yara,plusvic/yara,plusvic/yara,VirusTotal/yara,hillu/yara,cblichmann/yara,plusvic/yara