Simpler handling of errors when creating tbc variables

New field 'lua_State.ptbc' keeps to-be-closed variable until its
upvalue is created, so that it can be closed in case of a
memory-allocation error.

lstate.h

@@ -308,6 +308,7 @@ struct lua_State {
   int basehookcount;
   int hookcount;
   volatile l_signalT hookmask;
+  StkId ptbc;  /* pending to-be-closed variable */
 };