Bug: Buffer overflow in string concatenation

Even if the string fits in size_t, the whole size of the TString object
can overflow when we add the header.

lvm.c

@@ -661,7 +661,7 @@ void luaV_concat (lua_State *L, int total) {
       /* collect total length and number of strings */
       for (n = 1; n < total && tostring(L, s2v(top - n - 1)); n++) {
         size_t l = tsslen(tsvalue(s2v(top - n - 1)));
-        if (l_unlikely(l >= (MAX_SIZE/sizeof(char)) - tl)) {
+        if (l_unlikely(l >= MAX_SIZE - sizeof(TString) - tl)) {
           L->top.p = top - total;  /* pop strings to avoid wasting stack */
           luaG_runerror(L, "string length overflow");
         }