Commit cfdf4dc4 added an assertion that every WaitLatch() or similar
handles postmaster death.  One place did not, but was missed in
review and testing due to the need for an SSL connection.  Fix, by
asking for WL_EXIT_ON_PM_DEATH.

Reported-by: Christoph Berg
Discussion: https://postgr.es/m/20181124143845.GA15039%40msg.df7cb.de