gvariant tests: workaround libc/compiler "issue"

memcmp() is declared by glibc as follows:

  /* Compare N bytes of S1 and S2.  */
  extern int memcmp (const void *__s1, const void *__s2, size_t __n)
       __THROW __attribute_pure__ __nonnull ((1, 2));

despite the fact that it is valid to call it with a null pointer if the
size is zero.

gcc 4.9.0 contains a new optimisation that sees that we pass a pointer
to this function and concludes that it certainly must not be null,
removing a later check and thereby causing a crash.

We protect the invocation of memcmp() with a condition to prevent gcc
from making this false assumption (arguably under wrong advice from
glibc).