Fix NPE when submitting review with inline comments via REST

NPE occurs when submitting a review with inline comments in
the format:

  {
    "comments": {
      "test.txt": [
        {
          "line": 1,
          "message": "comment"
        },
      ]
    }
  }

The comma following the closing brace after the `"message": "comment"`
line seems to be causing an iterator with a null element to be
generated.

NPE does not occur when the comma is omitted.

To prevent the NPE remove the null element from the iterator.

Change-Id: I248739fce955522f7ee4a93441e48914342d3d7d

gerrit-server/src/main/java/com/google/gerrit/server/change/PostReview.java

@@ -282,6 +282,10 @@ public class PostReview implements RestModifyView<RevisionResource, ReviewInput>
       Iterator<Comment> listItr = list.iterator();
       while (listItr.hasNext()) {
         Comment c = listItr.next();
+        if (c == null) {
+          listItr.remove();
+          continue;
+        }
         if (c.line < 0) {
           throw new BadRequestException(String.format(
               "negative line number %d not allowed on %s",