debug/dwarf: skip over zero-length compilation units
debug/dwarf: skip over zero-length compilation units

DWARF sections generated by mingw-clang seem to include these
(not often - only one out of many in the binary that I am looking at).
Skipping over them, everything parses correctly.

This makes TestDefaultLinkerDWARF pass on windows/arm64.

Change-Id: Ie4a7daa1423f51cbc8c4aac88b1d27c3b52ee880
Reviewed-on: https://go-review.googlesource.com/c/go/+/312031
Trust: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>

golang/go,CAFxX/go,golang/go,nawawi/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,golang/go,golang/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,CAFxX/go