Check for invalid NonNull in isValidLiteralValue

Nil panic found with go-fuzz.

Signed-off-by: Jonathan Rudenberg <[email protected]>

Author: titanous

rules.go

@@ -1717,6 +1717,9 @@ func VariablesInAllowedPositionRule(context *ValidationContext) *ValidationRuleI
 func isValidLiteralValue(ttype Input, valueAST ast.Value) (bool, []string) {
 	// A value must be provided if the type is non-null.
 	if ttype, ok := ttype.(*NonNull); ok {
+		if e := ttype.Error(); e != nil {
+			return false, []string{e.Error()}
+		}
 		if valueAST == nil {
 			if ttype.OfType.Name() != "" {
 				return false, []string{fmt.Sprintf(`Expected "%v!", found null.`, ttype.OfType.Name())}

rules_default_values_of_correct_type_test.go

@@ -101,3 +101,7 @@ func TestValidate_VariableDefaultValuesOfCorrectType_ListVariablesWithInvalidIte
 				2, 40),
 		})
 }
+
+func TestValidate_VariableDefaultValuesOfCorrectType_InvalidNonNull(t *testing.T) {
+	testutil.ExpectPassesRule(t, graphql.DefaultValuesOfCorrectTypeRule, `query($g:e!){a}`)
+}