Similar to 3837b83f

, glibc memcmp is declared with the first two
arguments annotated as non-null via an attribute, which results in the
undefined behaviour sanitizer considering it to be UB to pass a null
pointer there (even if we are comparing 0 bytes, and hence not actually
dereferencing the pointer).

This shows up in /gvariant/serialiser/children when run with the
undefined behaviour sanitizer.

Signed-off-by: Simon McVittie <smcv@collabora.com>