No errors in 'luaO_pushvfstring'

Any call to 'va_start' must have a corresponding call to 'va_end';
so, functions called between them (luaO_pushvfstring in particular)
cannot raise errors.

ldebug.c

@@ -847,7 +847,8 @@ l_noret luaG_runerror (lua_State *L, const char *fmt, ...) {
   va_start(argp, fmt);
   msg = luaO_pushvfstring(L, fmt, argp);  /* format message */
   va_end(argp);
-  if (isLua(ci)) {  /* if Lua function, add source:line information */
+  if (msg != NULL && isLua(ci)) {  /* Lua function? (and no error) */
+    /* add source:line information */
     luaG_addinfo(L, msg, ci_func(ci)->p->source, getcurrentline(ci));
     setobjs2s(L, L->top.p - 2, L->top.p - 1);  /* remove 'msg' */
     L->top.p--;