fix lua crashing after an lua error

This fixes 797613f760

which changed the luaW_pcall code from.

 if(lua_pcall(..)) {
   ...
   lua_pop(L, 2);
   return;
 }
 lua_remove(L, error_handler_index);

to

 int r = lua_pcall(..);
 lua_remove(L, error_handler_index);
 if(r) {
   ...
   lua_pop(L, 2);
   return;
 }

so that three values were removed from the stack in case of an error which caused random segfaults later.

src/scripting/lua_common.cpp

@@ -983,7 +983,7 @@ bool luaW_pcall(lua_State *L, int nArgs, int nRets, bool allow_wml_error)
 		} else {
 			chat_message("Lua caught unknown exception", "");
 		}
-		lua_pop(L, 2);
+		lua_pop(L, 1);
 		return false;
 	}