fix dangling reference in foreach_key_value_pair

fix dangling reference in foreach_key_value_pair

When the user deletes the last entry in a bihash bucket, the bihash
infra frees the bucket's backing storage. If this happens under
clib_bihash_foreach_key_value_pair - and the freed bucket happens to
be the bucket being traversed - the resulting dangling reference can
easily make the wheels fall off.

Simple fix: if (bucket-is-now-empty) double-break.

Change-Id: Idc44247a82ed5d0ba548507b4a53d4c8503ba8bb
Signed-off-by: Dave Barach <dave@barachs.net>