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>