git-clean: fix off-by-one memory access when given no arguments

git-clean: fix off-by-one memory access when given no arguments

The "seen" variable is used by match_pathspec, and must have
as many elements as there are in the given pathspec. We
create the pathspec either from the command line arguments
_or_ from just the current prefix.

Thus allocating "seen" based upon just argc is wrong, since
if argc == 0, then we still have one pathspec, the prefix,
but we don't allocate any space in "seen".

Signed-off-by: Jeff King <peff@peff.net>
Tested-by: İsmail Dönmez <ismail@pardus.org.tr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>