yet another tweak to empty brigade checking on entry to core_input_filter():

yet another tweak to empty brigade checking on entry to core_input_filter():

  since APR_BRIGADE_EMPTY() assumes a non-empty brigade, we have to check
  before invoking that macro

  since APR_BRIGADE_EMPTY() can make a brigade empty, we have to check
  after invoking that macro

An obvious variation of this fix would be to change APR_BRIGADE_NORMALIZE()
to deal with empty brigades.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93293 13f79535-47bb-0310-9956-ffa450edef68