ignore empty lumps in W_ReadLump()

Do not even attempt to read empty lumps, because Windows' POSIX read()
implementation fails when passed a `NULL` buffer parameter, even though
the len parameter is also `0`.

src/w_wad.c

@@ -436,7 +436,7 @@ void W_ReadLump(int lump, void *dest)
 
   if (l->data)     // killough 1/31/98: predefined lump data
     memcpy(dest, l->data, l->size);
-  else
+  else if (l->size) // [FG] ignore empty lumps
     {
       int c;