commit -S: avoid invalid pointer with empty message

commit -S: avoid invalid pointer with empty message

While it is not recommended, fsck.c says:

	Not having a body is not a crime [...]

... which means that we cannot assume that the commit buffer
contains an empty line to separate header from body.  A commit
object with only a header without any body, not even without
a blank line after the header, is valid.

So let's tread carefully here.  strstr("\n\n") may find nothing
and return NULL.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>