t5562: do not depend on /dev/zero

It was reported [1] that NonStop platform does not have /dev/zero.

The test uses /dev/zero as a dummy input. Passing case (http-backed
failed because of too big input size) should not be reading anything
from it. If http-backend would erroneously try to read any data
returning EOF probably would be even safer than providing some
meaningless data.

Replace /dev/zero with /dev/null to avoid issues with platforms which do
not have /dev/zero.

[1] https://public-inbox.org/git/20190209185930.5256-4-randall.s.becker@rogers.com/Reported-by: NRandall S. Becker <rsbecker@nexbridge.com>
Signed-off-by: NMax Kirillov <max@max630.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t5562-http-backend-content-length.sh

@@ -150,7 +150,7 @@ test_expect_success 'CONTENT_LENGTH overflow ssite_t' '
 		GIT_HTTP_EXPORT_ALL=TRUE \
 		REQUEST_METHOD=POST \
 		CONTENT_LENGTH="$NOT_FIT_IN_SSIZE" \
-		git http-backend </dev/zero >/dev/null 2>err &&
+		git http-backend </dev/null >/dev/null 2>err &&
 	grep "fatal:.*CONTENT_LENGTH" err
 '