Handle nil file when creating a new issue

This can occur when there is a compilation issue into the code.

Change-Id: I8b028d454d417c33edc687dcffc68c92d18091d6
Signed-off-by: Cosmin Cojocar <ccojocar@google.com>

Author: ccojocar

analyzers/util.go

@@ -62,6 +62,9 @@ func newIssue(analyzerID string, desc string, fileSet *token.FileSet,
 	pos token.Pos, severity, confidence issue.Score,
 ) *issue.Issue {
 	file := fileSet.File(pos)
+	if file == nil {
+		return &issue.Issue{}
+	}
 	line := file.Line(pos)
 	col := file.Position(pos).Column