Correctly return 413 when an invalid chunk size is given on input.

- If get_chunk_size() returns a negative number, that probably implies
  an overflow.  So, create a 413 error and pass it to the output filters.
- Modify ap_discard_request_body() to return OK quickly if we're a subreq
  or our status code implies that we will be dropping the connection.
- Modify ap_die() so that if the new status implies that we will drop
  the connection, that we correctly indicate that we can not keepalive
  this connection.  (Without this, the error is returned, but the connection
  is not closed.)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95331 13f79535-47bb-0310-9956-ffa450edef68