fix(required-fields): don't throw on non-existent field (#65)

* Do not throw when referencing non-existent attr

* Test to reproduce thrown error

* Update CHANGELOG

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ### vNEXT
 
+- Fix `graphql/required-fields` throwing on non-existent field reference [Benjie](https://github.com/benjie) in [#65](https://github.com/apollographql/eslint-plugin-graphql/pull/65)
+
 ### v0.8.0
 
 - Updated `graphql` dependency to resolve test failures ([wording change](https://github.com/graphql/graphql-js/commit/ba401e154461bca5323ca9121c6dacaee10ebe88), no API change) [jnwng](https://github.com/jnwng)

src/rules.js

@@ -16,6 +16,9 @@ export function RequiredFields(context, options) {
   return {
     Field(node) {
       const def = context.getFieldDef();
+      if (!def) {
+        return;
+      }
       const { requiredFields } = options;
       requiredFields.forEach(field => {
         const fieldAvaliableOnType = def.type && def.type._fields && def.type._fields[field];

test/makeRule.js

@@ -796,7 +796,8 @@ const namedOperationsValidatorCases = {
 const requiredFieldsTestCases = {
   pass: [
     'const x = gql`query { allFilms { films { title } } }`',
-    'const x = gql`query { stories { id comments { text } } }`'
+    'const x = gql`query { stories { id comments { text } } }`',
+    'const x = gql`query { greetings { id, hello, foo } }`'
   ],
   fail: [
     {