Fix -Wshadow warnings

Checked that it is supported by gcc 4.2.1 (FreeBSD 9).

fixes #240
diff --git a/programs/test/udp_proxy.c b/programs/test/udp_proxy.c
index 645f94d..c49c46c 100644
--- a/programs/test/udp_proxy.c
+++ b/programs/test/udp_proxy.c
@@ -389,7 +389,7 @@
 
     while( cur < end )
     {
-        size_t len = ( ( cur[11] << 8 ) | cur[12] ) + 13;
+        len = ( ( cur[11] << 8 ) | cur[12] ) + 13;
 
         id = len % sizeof( dropped );
         ++dropped[id];