gtestutils: Add additional non-NULL check in g_assert_cmpmem()
gtestutils: Add additional non-NULL check in g_assert_cmpmem()

The compiler can’t work out from the combination of other conditions
that it’s not possible for (m2 == NULL) to hold true when memcmp() is
called, so add an explicit condition.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #1897

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib