MDEV-21362: Do not call memcmp on null pointers

Starting with commit 373443903b
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 fc168c3a5e (MDEV-15587).

storage/innobase/rem/rem0cmp.cc

@@ -448,8 +448,7 @@ cmp_data(
 	}
 
 	ulint len = std::min(len1, len2);
-
-	int cmp = memcmp(data1, data2, len);
+	int cmp = len ? memcmp(data1, data2, len) : 0;
 
 	if (cmp) {
 		return (cmp);