Starting with commit 37344390
we would invoke memcmp() unconditionally, even if the length is zero.
But, a call to memcmp() is undefined if any parameter is a null pointer,
even if the length is zero.

In the following tests, a null pointer is being passed to the comparison:
vcol.vcol_keys_innodb gcol.gcol_keys_innodb main.func_group_innodb
innodb.innodb_bug53592

cmp_data(): Keep WITH_UBSAN happy and avoid potential future bugs
in optimized builds, like the one addressed by
commit fc168c3a (MDEV-15587).