ssl: clear the error queue before SSL_read/write/accept()

If other modules or libraries do not clear the OpenSSL error queue after
a failed operation, other code that relies on SSL_get_error() -- in
particular, code that deals with SSL_ERROR_WANT_READ/WRITE logic -- will
malfunction later on. To prevent this, explicitly clear the error queue
before calls like SSL_read/write/accept().

PR: 60223
Submitted by: Paul Spangler <paul.spangler ni.com>

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1769332 13f79535-47bb-0310-9956-ffa450edef68