builtin/receive-pack: fix incorrect pointer arithmetic

If we had already processed the last newline in a push certificate, we would end up subtracting NULL from the end-of-certificate pointer when computing the length of the line. This would have resulted in an absurdly large length, and possibly a buffer overflow. Instead, subtract the beginning-of-certificate pointer from the end-of-certificate pointer, which is what's expected. Note that this situation should never occur, since not only do we require the certificate to be newline terminated, but the signature will only be read from the beginning of a line. Nevertheless, it seems prudent to correct it. Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>