list-objects-filter-options: do not over-strbuf_init

list-objects-filter-options: do not over-strbuf_init

The function gently_parse_list_objects_filter is either called with
errbuf=STRBUF_INIT or errbuf=NULL, but that function calls strbuf_init
when errbuf is not NULL. strbuf_init is only necessary if errbuf
contains garbage, and risks a memory leak if errbuf already has a
non-STRBUF_INIT state. It should be the caller's responsibility to make
sure errbuf is not garbage, since garbage content is easily avoidable
with STRBUF_INIT.

Signed-off-by: Matthew DeVore <matvore@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>