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/cryptsetup/cryptsetup.c

@@ -333,8 +333,7 @@ static int get_password(const char *vol, const char *src, usec_t until, bool acc
                 /* If the description string is simply the
                  * volume name, then let's not show this
                  * twice */
-                free(description);
-                description = NULL;
+                description = mfree(description);
         }
 
         if (mount_point && description)