Incorrect use of setobjs2s

[Daurnimator <quae@...>]

Looking through the lua source, I noticed in ldo.c:

setobjs2s(L, g->mainthread->top++, L->top - 1);  /* copy error obj. */

As far as I can see from comments in lobject.h, setobjs2s is meant to
only be used when copying to the same stack.