lf_to_crlf(): NUL-terminate msg_data::data

lf_to_crlf(): NUL-terminate msg_data::data

Through the rest of the file, the data member of struct msg_data is
kept NUL-terminated, and that fact is relied upon in a couple of
places.  Change lf_to_crlf() to preserve this invariant.

In fact, there are no execution paths in which lf_to_crlf() is called
and then its data member is required to be NUL-terminated, but it is
better to be consistent to prevent future confusion.

Document the invariant in the struct msg_data definition.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>