Fix crash in no-unused-prop-types when encountering mixed union and i…

…ntersection flow types (fixes #1806)

lib/rules/no-unused-prop-types.js

@@ -361,7 +361,7 @@ module.exports = {
         and property value. (key, value) => void
      */
     function iterateProperties(properties, fn) {
-      if (properties.length && typeof fn === 'function') {
+      if (properties && properties.length && typeof fn === 'function') {
         for (let i = 0, j = properties.length; i < j; i++) {
           const node = properties[i];
           const key = getKeyValue(node);

tests/lib/rules/no-unused-prop-types.js

@@ -4551,6 +4551,29 @@ ruleTester.run('no-unused-prop-types', rule, {
       errors: [{
         message: '\'defaultValue\' PropType is defined but prop is never used'
       }]
+    }, {
+      // Mixed union and intersection types
+      code: `
+        import React from 'react';
+        type OtherProps = {
+          firstname: string,
+          lastname: string,
+        } | {
+          fullname: string
+        };
+        type Props = OtherProps & {
+          age: number
+        };
+        class Test extends React.PureComponent<Props> {
+          render() {
+            return <div>Hello {this.props.firstname}</div>
+          }
+        }
+      `,
+      parser: 'babel-eslint',
+      errors: [{
+        message: '\'age\' PropType is defined but prop is never used'
+      }]
     }
 
     /* , {