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>