PostReview: Fix NPE on file level comments

The line member is optional and can be null for file level comments.

Change-Id: I77772b6f80772d5161eaa7db278ffc80c3560056

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

@@ -317,7 +317,7 @@ public class PostReview implements RestModifyView<RevisionResource, ReviewInput>
           listItr.remove();
           continue;
         }
-        if (c.line < 0) {
+        if (c.line != null && c.line < 0) {
           throw new BadRequestException(String.format(
               "negative line number %d not allowed on %s",
               c.line, path));