extensions: use UINT_MAX constants over open-coded bits (1/2)

~0 depends on the sizeof(int), so it is better to use UINT32_MAX.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
diff --git a/extensions/libxt_string.c b/extensions/libxt_string.c
index b440fc9..0408c23 100644
--- a/extensions/libxt_string.c
+++ b/extensions/libxt_string.c
@@ -57,7 +57,7 @@
 	struct xt_string_info *i = (struct xt_string_info *) m->data;
 
 	if (i->to_offset == 0)
-		i->to_offset = (u_int16_t) ~0UL;
+		i->to_offset = UINT16_MAX;
 }
 
 static void