internal/lsp: add an extra bounds check to avoid nil pointers

A nil pointer was reported to the golang-tools group (see
https://groups.google.com/g/golang-tools/c/JrNTz8I6ifo/m/tcJRpek-AAAJ).
I think this bounds check should address it.

Updates golang/go#34433

Change-Id: I87352c269c65c844c86ebe9ee3fd2d041cc49ee9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/227770
Run-TryBot: Rebecca Stambler <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: stamblerre

internal/lsp/source/format.go

@@ -245,9 +245,12 @@ func trimToImports(fset *token.FileSet, f *ast.File, src []byte) ([]byte, int) {
 	if nextLine := fset.Position(end).Line + 1; tok.LineCount() >= nextLine {
 		end = fset.File(f.Pos()).LineStart(nextLine)
 	}
+	if start > end {
+		return nil, 0
+	}
 
 	startLineOffset := fset.Position(start).Line - 1 // lines are 1-indexed.
-	return src[fset.Position(firstImport.Pos()).Offset:fset.Position(end).Offset], startLineOffset
+	return src[fset.Position(start).Offset:fset.Position(end).Offset], startLineOffset
 }
 
 // trimToFirstNonImport returns src from the beginning to the first non-import