Fixed bug of long strings in binary chunks

When "undumping" a long string, the function 'loadVector' can call the
reader function, which can run the garbage collector, which can collect
the string being read. So, the string must be anchored during the call
to 'loadVector'.

lundump.c

@@ -120,7 +120,10 @@ static TString *loadStringN (LoadState *S, Proto *p) {
   }
   else {  /* long string */
     ts = luaS_createlngstrobj(L, size);  /* create string */
+    setsvalue2s(L, L->top, ts);  /* anchor it ('loadVector' can GC) */
+    luaD_inctop(L);
     loadVector(S, getstr(ts), size);  /* load directly in final place */
+    L->top--;  /* pop string */
   }
   luaC_objbarrier(L, p, ts);
   return ts;