Witold Kręcicki authored May 31, 2019 and Evan Hunt committed May 31, 2019

There's a small possibility of race between udp dispatcher and
socket code - socket code can still hold internal reference to a
socket while dispatcher calls isc_socket_open, which can cause
an assertion failure. Fix it by relaxing the assertion test, and
instead simply locking the socket in isc_socket_open.