tree-wide: introduce mfree()

Pretty trivial helper which wraps free() but returns NULL, so we can
simplify this:
        free(foobar);
        foobar = NULL;
to this:
        foobar = mfree(foobar);

src/basic/util.h

@@ -84,6 +84,11 @@ bool streq_ptr(const char *a, const char *b) _pure_;
 
 #define malloc0(n) (calloc((n), 1))
 
+static inline void *mfree(void *memory) {
+        free(memory);
+        return NULL;
+}
+
 static inline const char* yes_no(bool b) {
         return b ? "yes" : "no";
 }