Towers: reject descriptions with odd characters at the end

Towers' new_game() causes an assertion failure on game description
strings that contain spurious characters after a valid description, so
validate_desc() should also refuse such a description.  The problem
could be demonstrated by editing the game description in the
"Specific" dialogue box to add a '!' at the end, which caused
"new_game: Assertion `!*p' failed.".

towers.c

@@ -893,6 +893,7 @@ static const char *validate_desc(const game_params *params, const char *desc)
 	    return "Too much data to fit in grid";
     }
 
+    if (*p) return "Rubbish at end of game description";
     return NULL;
 }