Move a check for an empty brigade to the start of core input filter

to avoid segfaults.

Using prefork MPM and APR_POOL_DEBUG and ElectricFence, I found a
couple of paths where weren't able to report APR_EOF all the way
back up the call tree, leading us back into core_input_filter with
an empty brigade.

Some of the paths in core_input_filter handled it, some didn't.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@92835 13f79535-47bb-0310-9956-ffa450edef68