Fix panic in t.Errorf check if the typechecker fails.

Fixes #116.
diff --git a/lint.go b/lint.go
index 5749484..8182c0b 100644
--- a/lint.go
+++ b/lint.go
@@ -1046,8 +1046,13 @@
 			return true
 		}
 		isErrorsNew := isPkgDot(ce.Fun, "errors", "New")
+		var isTestingError bool
 		se, ok := ce.Fun.(*ast.SelectorExpr)
-		isTestingError := ok && se.Sel.Name == "Error" && f.pkg.typeOf(se.X).String() == "*testing.T"
+		if ok && se.Sel.Name == "Error" {
+			if typ := f.pkg.typeOf(se.X); typ != nil {
+				isTestingError = typ.String() == "*testing.T"
+			}
+		}
 		if !isErrorsNew && !isTestingError {
 			return true
 		}