SIGSEGV in Sanitizer INTERCEPTOR of strstr function.

This is a segmentation fault in INTERCEPTOR function on a special edge
case of strstr libc call. When 'Haystack'(main string to be examined) is
NULL and 'needle'(sub-string to be searched in 'Haystack') is an empty
string then it hits a SEGV while using sanitizers and as a 'string not
found' case otherwise.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D115919