Emit JSON errors from PHP Parser

The PHP parser had a bug here in that it presumed JSON encoding would
succeed (and there are reasons it might not). This checks the return
value of `json_encode` and emits a relevant error message if
appropriate. Before this, a triggering case would result in empty output
and an unhelpful error message from `JSON.parse` upstream in the Ruby
code.

Author: wfleming

vendor/php-parser/parser.php

@@ -13,8 +13,13 @@
 
   $serializer = new PhpParser\Serializer\JSON;
   $nodes = $serializer->serialize($stmts);
-  echo json_encode($nodes);
-
+  $json = json_encode($nodes);
+  if (false === $json) {
+    fwrite(STDERR, "Parse Error: JSON encoding failed: ".json_last_error_msg()."\n");
+    exit(1);
+  } else {
+    echo $json;
+  }
 } catch (PHPParser\Error $e) {
   fwrite(STDERR, "Parse Error: ".$e->getMessage()."\n");
   exit(1);