Fix describe --tags --long so it does not segfault

If we match a lightweight (non-annotated tag) as the name to
output and --long was requested we do not have a tag, nor do
we have a tagged object to display.  Instead we must use the
object we were passed as input for the long format display.

Reported-by: Mark Burton <markb@ordern.com>
Backtraced-by: Mikael Magnusson <mikachu@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-describe.c

@@ -204,7 +204,7 @@ static void describe(const char *arg, int last_one)
 		 */
 		display_name(n);
 		if (longformat)
-			show_suffix(0, n->tag->tagged->sha1);
+			show_suffix(0, n->tag ? n->tag->tagged->sha1 : sha1);
 		printf("\n");
 		return;
 	}

t/t6120-describe.sh

@@ -139,4 +139,6 @@ check_describe "test1-lightweight-*" --tags --match="test1-*"
 
 check_describe "test2-lightweight-*" --tags --match="test2-*"
 
+check_describe "test2-lightweight-*" --long --tags --match="test2-*" HEAD^
+
 test_done