Suppress uninitialized-variable warning.

Suppress uninitialized-variable warning.

In the wake of commit 80aa9848b, a few compilers think that
postgresAcquireSampleRowsFunc's "reltuples" might be used
uninitialized.  The logic is visibly correct, both before
and after that change; presumably what happened here is that
the previous presence of a setjmp() in the function stopped
them from attempting any flow analysis at all.  Add a dummy
initialization to silence the warning.

Reported-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAExHW5tkerCufA_F6oct5dMJ61N+yVrVgYXL7M8dD-5_zXjrDw@mail.gmail.com