Limit access end-of-struct warning a bit

Only warn if the struct has a non-zero size.  You can't create objects
of zero-sized structs, but they can be used inside sizeof (e.g.
"sizeof (struct {int :0;})".  The warning would always trigger for these,
but as no objects can be created no accesses can ever happen.

tccgen.c

@@ -3471,7 +3471,7 @@ static void struct_layout(CType *type, AttributeDef *ad)
     type->ref->c = (c + (pcc ? (bit_pos + 7) >> 3 : 0)
 		    + maxalign - 1) & -maxalign;
     type->ref->r = maxalign;
-    if (offset + size > type->ref->c)
+    if (offset + size > type->ref->c && type->ref->c)
         tcc_warning("will touch memory past end of the struct (internal limitation)");
 }