Fix a fun heisenbug in memoize_protover_flags()
After we clear the protover map for getting full, we need to
re-create it, since we are about to use it.
This is a bugfix for bug 28558. It is a bugfix for the code from
ticket 27225, which is not in any released Tor. Found by Google
OSS-Fuzz, as issue 11475.