add_to_index(): free unused cache-entry

add_to_index(): free unused cache-entry

We allocate a cache-entry pretty early in the function and then
decide either not to do anything when we are pretending to add, or
add it and then get an error (another possibility is obviously to
succeed).

When pretending or failing to add, we forgot to free the
cache-entry.

Noticed during a discussion on Stefan's patch to change the coding
style without fixing the issue ;-)

Signed-off-by: Junio C Hamano <gitster@pobox.com>