[sanitizer] Fix crash in getgrnam_r and similar interceptors.

When no matching record is found, getgrnam_r return 0 but sets result to NULL.
Should fix PR19734.



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@208773 91177308-0d34-0410-b5e6-96231b3b80d8