apply: use starts_with() in gitdiff_verify_name()

Avoid running over the end of line -- a C string whose length is not
known to this function -- by using starts_with() instead of memcmp(3)
for checking if it starts with "/dev/null".  Also simply include the
newline in the string constant to compare against.  Drop a comment that
just states the obvious.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: rscharfe

apply.c

@@ -970,8 +970,7 @@ static int gitdiff_verify_name(struct apply_state *state,
 		}
 		free(another);
 	} else {
-		/* expect "/dev/null" */
-		if (memcmp("/dev/null", line, 9) || line[9] != '\n')
+		if (!starts_with(line, "/dev/null\n"))
 			return error(_("git apply: bad git-diff - expected /dev/null on line %d"), state->linenr);
 	}