ap_get_mime_headers: tighten up the null termination of header line which is

ap_get_mime_headers: tighten up the null termination of header line which is
too long.  getline can return a smaller length that what it actually read in
that case.

The check for len > limit_fieldsize isn't needed, but we do need to insure
that getline actually allocated a buffer (and set len) in the case where
the first socket input buffer is already bigger than the limit.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@97863 13f79535-47bb-0310-9956-ffa450edef68