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 4f3dcc2 (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: Junio C Hamano <gitster@pobox.com>

t/t7007-show.sh

@@ -0,0 +1,20 @@
+#!/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