diff: fix a possible null pointer dereference

The condition in the ternary operator was wrong, hence the wrong char
pointer could be used as the parameter for show_submodule_summary.
one->path may be null, but we definitely need a non null path given
to the function.

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
Acked-By: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -2252,7 +2252,7 @@ static void builtin_diff(const char *name_a,
 			(!two->mode || S_ISGITLINK(two->mode))) {
 		const char *del = diff_get_color_opt(o, DIFF_FILE_OLD);
 		const char *add = diff_get_color_opt(o, DIFF_FILE_NEW);
-		show_submodule_summary(o->file, one ? one->path : two->path,
+		show_submodule_summary(o->file, one->path ? one->path : two->path,
 				line_prefix,
 				one->sha1, two->sha1, two->dirty_submodule,
 				meta, del, add, reset);