record_author_date(): fix memory leak on malformed commit

record_author_date(): fix memory leak on malformed commit

If we hit the end-of-header without finding an "author"
line, we just return from the function. We should jump to
the fail_exit path to clean up the buffer that we may have
allocated.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>