internal/lsp: handle empty buffers in template parsing

Fixes golang/vscode-go#1540

Change-Id: I64514915d555bf0c0ff791b213bb834e9e99a0af
Reviewed-on: https://go-review.googlesource.com/c/tools/+/324758
Trust: Rebecca Stambler <[email protected]>
Trust: Peter Weinberger <[email protected]>
Run-TryBot: Rebecca Stambler <[email protected]>
gopls-CI: kokoro <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Peter Weinberger <[email protected]>
(cherry picked from commit 4abb1e2)
Reviewed-on: https://go-review.googlesource.com/c/tools/+/324761
Reviewed-by: Robert Findley <[email protected]>

Author: stamblerre

internal/lsp/template/parse.go

@@ -28,6 +28,7 @@ import (
 	"golang.org/x/tools/internal/lsp/protocol"
 	"golang.org/x/tools/internal/lsp/source"
 	"golang.org/x/tools/internal/span"
+	errors "golang.org/x/xerrors"
 )
 
 var (
@@ -86,6 +87,10 @@ func parseBuffer(buf []byte) *Parsed {
 		buf:   buf,
 		check: -1,
 	}
+	if len(buf) == 0 {
+		ans.ParseErr = errors.New("empty buffer")
+		return ans
+	}
 	// how to compute allAscii...
 	for _, b := range buf {
 		if b >= utf8.RuneSelf {