Address GCC Bug 95189 memcmp wrongly stripped like strcmp

Address GCC Bug 95189 memcmp wrongly stripped like strcmp

As documented in Russell O'Connor's blog, Heimdal when compiled with
some versions of gcc 9 and 10 would generate incorrect behaviors from
_gssapi_verify_mic_arcfour(), _gssapi_unwrap_arcfour(),
_gssapi_unwrap_iov_arcfour() and _gssapi_unwrap_iov_arcfour().

As a result of the bug, code of the form

   if (memcmp(a, "\x00\x00\x00\x00"))

and

   cmp = memcmp(a, "\x00\x00\x00\x00")

will be compiled as if it were written as

   if (strcmp(a, "\x00\x00\x00\x00"))

and

   cmp = strcmp(a, "\x00\x00\x00\x00")

but not

   if (memcmp(a, "\x00\x00\x00\x00") != 0)

and

   cmp = (memcmp(a, "\x00\x00\x00\x00") != 0)

Bad code is generated whenever one of the parameters to memcmp()
is a constant with at least one NUL in the first four octets and
the return value is used immediated without a boolean comparison.

The gcc bug 95189 has since been fixed.

This change applies a defensive programming technique to avoid
the broken code generation.

Change-Id: I1db2a561735317cb6cac66a0ec9caf5443e65e03
Link: https://r6.ca/blog/20200929T023701Z.html
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95189
Reported-by: Buck Huppmann (buckh@pobox.com) <buckh@pobox.com>