Fix stack handling on errors for the mouse grabber

This one actually uses the return value from the function (where's the
consistency?!), so the code is fixed to only pop the return value if there is
actually one.

Signed-off-by: Uli Schlachter <psychon@znc.in>

event.c

@@ -122,10 +122,11 @@ event_handle_mousegrabber(int x, int y, uint16_t mask)
         {
             warn("Stopping mousegrabber.");
             luaA_mousegrabber_stop(L);
-        }
-        else if(!lua_isboolean(L, -1) || !lua_toboolean(L, -1))
+        } else {
+            if(!lua_isboolean(L, -1) || !lua_toboolean(L, -1))
                 luaA_mousegrabber_stop(L);
             lua_pop(L, 1);  /* pop returned value */
+        }
         return true;
     }
     return false;