memory: plug a leak, by actually freeing a discarded undo struct

This fixes https://savannah.gnu.org/bugs/?53269.
Reported-by: Ashish Kunwar <dorkerdevil280@gmail.com>

The leak was mistakenly introduced by commit dc3618a1
-- probably as a leftover of testing things.

src/text.c

@@ -1184,6 +1184,7 @@ void discard_until(const undo *thisitem, openfilestruct *thefile, bool keep)
 			free(group);
 			group = next;
 		}
+		free(dropit);
 		dropit = thefile->undotop;
 	}