upload-pack: don't send null character in abort message to the client

upload-pack: don't send null character in abort message to the client

Since 583b7ea31b (upload-pack/fetch-pack: support side-band
communication, 2006-06-21) the abort message sent by upload-pack in
case of possible repository corruption ends with a null character.
This can be seen in several test cases in 't5530-upload-pack-error.sh'
where 'grep <pattern> output.err' often reports "Binary file
output.err matches" because of that null character.

The reason for this is that the abort message is defined as a string
literal, and we pass its size to the send function as
sizeof(abort_msg), which also counts the terminating null character.

Use strlen() instead to avoid sending that terminating null character.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>