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: 992e818
GitHub-Pull-Request: #265
Reviewed-on: https://go-review.googlesource.com/c/tools/+/281092
Reviewed-by: Peter Weinberger <pjw@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
Trust: Rebecca Stambler <rstambler@golang.org>

internal/lsp/server.go

@@ -115,8 +115,9 @@ func (s *Server) workDoneProgressCancel(ctx context.Context, params *protocol.Wo
 }
 
 func (s *Server) nonstandardRequest(ctx context.Context, method string, params interface{}) (interface{}, error) {
-	paramMap := params.(map[string]interface{})
-	if method == "gopls/diagnoseFiles" {
+	switch method {
+	case "gopls/diagnoseFiles":
+		paramMap := params.(map[string]interface{})
 		for _, file := range paramMap["files"].([]interface{}) {
 			snapshot, fh, ok, release, err := s.beginFileRequest(ctx, protocol.DocumentURI(file.(string)), source.UnknownKind)
 			defer release()