builtin-mailinfo.c: check error status from rewind and ftruncate

A recent "cut at scissors" implementation rewinds and truncates
the output file to store the message when it sees a scissors mark,
but it did not check if these library calls succeeded.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
[sp: Use fseek as rewind returns void]
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>