internal/span: return error when on spans with invalid starts
internal/span: return error when on spans with invalid starts

Fixes golang/go#30155

Change-Id: I7f9dbec6fdfa92654f58627ef44ee4bfe2870fa6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/169440
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Reviewed-by: Ian Cottrell <52376736e5e84615ec49f9c1843b8acc178b1d8d@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools