remote.c: don't dereference NULL in freeing loop

remote.c: don't dereference NULL in freeing loop

Fix a bug in fd3cb0501e1 (remote: move static variables into
per-repository struct, 2021-11-17) where we'd free(remote->pushurl[i])
after having NULL'd out remote->pushurl. itself. We free
"remote->pushurl" in the next "for"-loop, so doing this appears to
have been a copy/paste error.

Before this change GCC 12's -fanalyzer would correctly note that we'd
dereference NULL in this case, this change fixes that:

	remote.c: In function ‘remote_clear’:
	remote.c:153:17: error: dereference of NULL ‘*remote.pushurl’ [CWE-476] [-Werror=analyzer-null-dereference]
	  153 |                 free((char *)remote->pushurl[i]);
	      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	      [...]

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>