cache: don't check for match with no key

cache: don't check for match with no key

We call open_slot() from cache_ls() without a key since we simply want
to read the path out of the header.  Should the file happen to contain
an empty key then we end up calling memcmp() with NULL and a non-zero
length.  Fix this by assigning slot->match only if a key is set, which
is always will be in the code paths where we use slot->match.

Coverity-id: 13807
Signed-off-by: John Keeping <john@keeping.me.uk>