http.c: fix an invalid free()

http.c: fix an invalid free()

Remove a free() on the static buffer returned by sha1_file_name().

While we're at it, replace xmalloc() calls on the structs
http_(object|pack)_request with xcalloc() so that pointers in the
structs get initialized to NULL. That way, free()'s are safe - for
example, a free() on the url string member when aborting.

This fixes an invalid free().

Reported-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Helped-by: Jeff King peff@peff.net
Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>