Fix a seg fault. The problem is easy to explain. On a HEAD request, Good

Fix a seg fault.  The problem is easy to explain.  On a HEAD request, Good
handlers will send their data down the filter stack, but 1.3 handlers will
just return, giving us a Content-Length of 0.  Since we can't send a C-L
of 0 just because it is a HEAD request, we search the headers_out table
for a 0 C-L if it is a HEAD request.  The problem is that some filters
will not allow (includes_filter) a C-L to be computed, so we end up without
a C-L header in headers_out.  Thus, when we do a strcmp against the header
value and "0", we seg fault, because the header value is NULL.

To fix this, we grab the element from the header table, and make sure it
isn't NULL before doing the strcmp.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@87870 13f79535-47bb-0310-9956-ffa450edef68