Create a message descriptor type.

This replaces the sentinel at the end of the pb_field_t
array for each message type.

git-svn-id: https://svn.kapsi.fi/jpa/nanopb-dev@1007 e3a754e5-d11d-0410-8d38-ebb782a927b9

tests/test_decode_callbacks.c

@@ -84,8 +84,8 @@ int main()
     testmessage.fixed64value.funcs.decode = &print_fixed64;
     testmessage.fixed64value.arg = "fixed64value: %lld\n";
     
-    if (!pb_decode(&stream, TestMessage_fields, &testmessage))
+    if (!pb_decode(&stream, TestMessage_msg, &testmessage))
         return 1;
     
     return 0;
-}
+}