platform: add NULL check in inline nmp_object_unref() function

This allows the compiler to see that this function does nothing for %NULL.
That is not so unusual, as we use nm_auto_nmpobj to free objects. Often
the compiler can see that these pointers are %NULL.

src/platform/nmp-object.h

@@ -632,9 +632,11 @@ nmp_object_ref (const NMPObject *obj)
 static inline void
 nmp_object_unref (const NMPObject *obj)
 {
-	nm_assert (!obj || NMP_OBJECT_IS_VALID (obj));
+	if (obj) {
+		nm_assert (NMP_OBJECT_IS_VALID (obj));
 
 		nm_dedup_multi_obj_unref ((const NMDedupMultiObj *) obj);
+	}
 }
 
 #define nm_clear_nmp_object(ptr) \