udp: fix udp_local length errors accounting

In case of UDP length errors in udp_local node, these errors are
being lost and incomplete header may be advanced by wrong offset.
Fix it with only full packets processing and explicit error set
otherwise. Also, optimize two buffer loop perfomance into fast
path with both buffers are ok and slow path with one or none.

Type: fix
Change-Id: I6b7edc3eb5593981e55d7ae20d753c0fd1549d86
Signed-off-by: Vladislav Grishenko <themiron@yandex-team.ru>