Fix OMG bug in modbus_mapping_free not freeing memory

Thanks to Stefan Finzel for the bug report

src/modbus.c

@@ -1561,7 +1561,7 @@ modbus_mapping_t* modbus_mapping_new(int nb_bits, int nb_input_bits,
 /* Frees the 4 arrays */
 void modbus_mapping_free(modbus_mapping_t *mb_mapping)
 {
-    if (mb_mapping != NULL) {
+    if (mb_mapping == NULL) {
         return;
     }