x/debug/dwarf: don't panic on unsupported type tags

Invoking Data.Type on an offset that doesn't correspond to a known type
tag should not result in a panic. Ensure that a type tag was matched and
exit early if not.

Change-Id: I1d490e2e131b5376b3ecedba8d871b513736ad66
Reviewed-on: https://go-review.googlesource.com/61070
Reviewed-by: John Dethridge <jcd@golang.org>