Fixed bug: invalid 'oldpc' when returning to a function

The field 'L->oldpc' is not always updated when control returns to a
function; an invalid value can seg. fault when computing 'changedline'.
(One example is an error in a finalizer; control can return to
'luaV_execute' without executing 'luaD_poscall'.) Instead of trying to
fix all possible corner cases, it seems safer to be resilient to invalid
values for 'oldpc'. Valid but wrong values at most cause an extra call
to a line hook.

lvm.c

@@ -1794,7 +1794,7 @@ void luaV_execute (lua_State *L, CallInfo *ci) {
         ProtectNT(luaT_adjustvarargs(L, GETARG_A(i), ci, cl->p));
         if (trap) {
           luaD_hookcall(L, ci);
-          L->oldpc = pc + 1;  /* next opcode will be seen as a "new" line */
+          L->oldpc = 1;  /* next opcode will be seen as a "new" line */
         }
         updatebase(ci);  /* function has new base after adjustment */
         vmbreak;