i965: Fix memmem compiler warnings.

i965: Fix memmem compiler warnings.

gcc is throwing this warning in my meson build:

../src/intel/compiler/brw_eu_validate.c:50:11: warning
argument 1 null where non-null expected [-Wnonnull]
    return memmem(haystack.str, haystack.len,
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                  needle.str, needle.len) != NULL;
                                  ~~~~~~~~~~~~~~~~~~~~~~~

The first check for CONTAINS has a NULL error_msg.str and 0 len.  The
glibc implementation will exit without looking at any haystack bytes if
haystack.len < needle.len, so this was safe, but silence the warning
anyway by guarding against implementation variablility.

Fixes: 122ef3799d56 ("i965: Only insert error message if not already present")
Reviewed-by: Matt Turner <mattst88@gmail.com>