Fix package check in error-strings rule (#610) (#611)

Co-authored-by: SalvadorC <salvadorcavadini+github@gmail.com>

rule/error-strings.go

@@ -105,6 +105,9 @@ func (w lintErrorStrings) match(expr *ast.CallExpr) bool {
 	}
 	// retrieve the package
 	id, ok := sel.X.(*ast.Ident)
+	if !ok {
+		return false
+	}
 	functions, ok := w.errorFunctions[id.Name]
 	if !ok {
 		return false

testdata/golint/error-strings.go

@@ -16,5 +16,9 @@ func g(x int) error {
 	err = fmt.Errorf("Newlines are really fun\n") // MATCH /error strings should not be capitalized or end with punctuation or a newline/
 	err = errors.New(`too much stuff.`)           // MATCH /error strings should not be capitalized or end with punctuation or a newline/
 	err = errors.New("This %d is too low", x)     // MATCH /error strings should not be capitalized or end with punctuation or a newline/
+
+	// Non-regression test for issue #610
+	d.stack.Push(from)
+
 	return err
 }