mailinfo: avoid segfault when can't open files

mailinfo: avoid segfault when can't open files

If <msg> or <patch> files can't be opened, then mailinfo() returns an
error before it even initializes mi->p_hdr_data or mi->s_hdr_data.
When cmd_mailinfo() then calls clear_mailinfo(), we dereference the
NULL pointers trying to free their contents.

Signed-off-by: Juan F. Codagnone <redacted>
Reviewed-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>