internal/lsp: avoid panic during interface assertion
internal/lsp: avoid panic during interface assertion

This change fixes a panic in the non-standard request.

Fixes golang/go#43462

Change-Id: I216a56f96ca6159cb9b102183ba3a3eddd186889
GitHub-Last-Rev: 992e818f8a57112b436b831a96b6a9a4fcfa7355
GitHub-Pull-Request: golang/tools#265
Reviewed-on: https://go-review.googlesource.com/c/tools/+/281092
Reviewed-by: Peter Weinberger <dd140fb9530d791057dd8833ea6768ff65fcde71@google.com>
Reviewed-by: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Trust: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>

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