libyara/strutils.c: fix potential null pointer dereference found by cppcheck
libyara/strutils.c: fix potential null pointer dereference found by cppcheck

libyara/strutils.c:236:18: warning: Either the condition 'haystack==NULL'
is redundant or there is pointer arithmetic with NULL pointer.

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