internal/lsp: fix panic when file deleted

Change-Id: I6eb7e5ae7fb9de74d5a3fb42031adb4e933d99d8
Reviewed-on: https://go-review.googlesource.com/c/tools/+/181680
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Ian Cottrell <iancottrell@google.com>

internal/lsp/cache/parse.go

@@ -143,7 +143,7 @@ func (imp *importer) parseFiles(filenames []string, ignoreFuncBodies bool) ([]*a
 	// Eliminate nils, preserving order.
 	var o int
 	for _, f := range parsed {
-		if f != nil {
+		if f.file != nil {
 			parsed[o] = f
 			o++
 		}