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

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>