pretty: fix leaking key/value separator buffer

pretty: fix leaking key/value separator buffer

The `format_set_trailers_options()` function is responsible for parsing
a custom pretty format for trailers. It puts the parsed options into a
`struct process_trailer_options` structure, while the allocated memory
required for this will be put into separate caller-provided arguments.
It is thus the caller's responsibility to free the memory not via the
options structure, but via the other parameters.

While we do this alright for the separator and filter keys, we do not
free the memory associated with the key/value separator. Fix this to
plug this memory leak.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>