Fix crash when a Lua tool Perform callback throws an error

Broken since 00ec4e0754, when the Perform callback was added. It's a bad idea to pop return values that never got pushed because the function that was supposed to push them threw an error.

src/lua/LuaTools.cpp

@@ -91,12 +91,15 @@ static int luaPerformWrapper(SimTool *tool, Simulation *sim, Particle *cpart, in
 			lsi->Log(CommandInterface::LogError, "In perform func: " + LuaGetError());
 			lua_pop(L, 1);
 		}
+		else
+		{
 			if (lua_isboolean(L, -1))
 			{
 				ok = lua_toboolean(L, -1);
 			}
 			lua_pop(L, 1);
 		}
+	}
 	return ok;
 }