Merge r1206850:

- Add some error handling for writing to the output filter chain.
- Add some ap_assert()s for error cases that probably should not happen (and
  the following code would break).
  These fix some compiler warnings about "variable 'rv' set but not used".
- Prevent a leak of a bucket in one error case.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1209736 13f79535-47bb-0310-9956-ffa450edef68