gcc-7 used with a sufficient optimization level complains about warnings
around do_to_timestamp() regarding the initialization and handling of
some of its variables.  Recent commits 66c74f8b and d589f944 made things
made the interface more confusing, so document which variables are
always expected and initialize properly the optional ones when they are
set.

Author: Andrey Lepikhov, Michael Paquier
Discussion: https://postgr.es/m/a7e28b83-27b1-4e1c-c76b-4268c4b785bc@postgrespro.ru