Toolgun Think fixes (#1547) - Check owner validity - fixes rare edge case of the owner being invalid when Think is called and cases where the owner is an NPC - Check last tool object validity before releasing its entity - fixes error when switching from invalid mode to another invalid mode - Remove a return when a mode switch occurs and the last mode was invalid, causing the first Think after a mode switch to be skipped - fixes garrysmod-issues/issues/3682