Improve DBC in isc_mem_free

Unlike standard free(), isc_mem_free() is not a no-op when passed a
NULL pointer. For size accounting purposes it calls sallocx(), which
crashes when passed a NULL pointer. To get more helpful diagnostics,
REQUIRE() that the pointer is not NULL so that when the programmer
makes a mistake they get a backtrace that shows what went wrong.

lib/isc/mem.c

@@ -981,6 +981,7 @@ isc__mem_free(isc_mem_t *ctx, void *ptr FLARG) {
 	size_t size = 0;
 
 	REQUIRE(VALID_CONTEXT(ctx));
+	REQUIRE(ptr != NULL);
 
 	size = sallocx(ptr, 0);