sockfd cache: fix a uninit warning

sockfd_cache.c:379:16: warning: ‘idx’ may be used uninitialized in this function [-Wuninitialized]
...

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>