go/analysis/analysistest: avoid nil panic from an invalid token.Pos

See failures in CL 638395

Change-Id: I03ea2ab77fd71707eb9fd6c2dd53ab553f5d8563
GitHub-Last-Rev: 39e217e
GitHub-Pull-Request: #549
Reviewed-on: https://go-review.googlesource.com/c/tools/+/638415
Auto-Submit: Robert Griesemer <[email protected]>
Commit-Queue: Alan Donovan <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: mateusz834

go/analysis/analysistest/analysistest.go

@@ -191,11 +191,20 @@ func RunWithSuggestedFixes(t Testing, dir string, a *analysis.Analyzer, patterns
 							act.Analyzer.Name, start, end)
 						continue
 					}
-					file, endfile := act.Package.Fset.File(start), act.Package.Fset.File(end)
-					if file == nil || endfile == nil || file != endfile {
+					file := act.Package.Fset.File(start)
+					if file == nil {
+						t.Errorf("diagnostic for analysis %v contains Suggested Fix with malformed start position %v", act.Analyzer.Name, start)
+						continue
+					}
+					endFile := act.Package.Fset.File(end)
+					if endFile == nil {
+						t.Errorf("diagnostic for analysis %v contains Suggested Fix with malformed end position %v", act.Analyzer.Name, end)
+						continue
+					}
+					if file != endFile {
 						t.Errorf(
 							"diagnostic for analysis %v contains Suggested Fix with malformed spanning files %v and %v",
-							act.Analyzer.Name, file.Name(), endfile.Name())
+							act.Analyzer.Name, file.Name(), endFile.Name())
 						continue
 					}
 					if _, ok := fileContents[file]; !ok {