Improve style of two code paths

Improve style of two code paths

In execGrouping.c, execTuplesMatchPrepare() was doing a memory
allocation that was not necessary when the number of columns was 0.
In foreign.c, pg_options_to_table() was assigning twice a variable to
the same value.

Author: Ranier Vilela
Discussion: https://postgr.es/m/CAEudQAqup0agbSzMjSLSTn=OANyCzxENF1+HrSYnr3WyZib7=Q@mail.gmail.com