git-show: do not segfault when showing a bad tag

When a tag points at a bad or nonexistent object, we should diagnose the
breakage and exit.  An earlier commit 4f3dcc27 (Fix 'git show' on signed
tag of signed tag of commit, 2008-07-01) lost this check and made it
segfault instead; not good.

This fixes it.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-log.c

@@ -356,7 +356,13 @@ int cmd_show(int argc, const char **argv, const char *prefix)
 					t->tag,
 					diff_get_color_opt(&rev.diffopt, DIFF_RESET));
 			ret = show_object(o->sha1, 1, &rev);
-			objects[i].item = parse_object(t->tagged->sha1);
+			if (ret)
+				break;
+			o = parse_object(t->tagged->sha1);
+			if (!o)
+				ret = error("Could not read object %s",
+					    sha1_to_hex(t->tagged->sha1));
+			objects[i].item = o;
 			i--;
 			break;
 		}

t/t7007-show.sh

+#!/bin/sh
+
+test_description='git show'
+
+. ./test-lib.sh
+
+test_expect_success setup '
+	echo hello world >foo &&
+	H=$(git hash-object -w foo) &&
+	git tag -a foo-tag -m "Tags $H" $H &&
+	HH=$(expr "$H" : "\(..\)") &&
+	H38=$(expr "$H" : "..\(.*\)") &&
+	rm -f .git/objects/$HH/$H38
+'
+
+test_expect_success 'showing a tag that point at a missing object' '
+	test_must_fail git --no-pager show foo-tag
+'
+
+test_done