Fix uninitialized variable access (src/backend/utils/mmgr/freepage.c)

Hi,

The function FreePageManagerPutInternal can access an uninitialized
variable,
if the following conditions occur:

1. fpm->btree_depth != 0
2. relptr_off == 0 inside function (FreePageBtreeSearch)

Perhaps this is a rare situation, but I think it's worth preventing.

/* Search the btree. */
FreePageBtreeSearch(fpm, first_page, &result);
Assert(!result.found);
if (result.index > 0) /* result.index is garbage or invalid here) */

regards,
Ranier Vilela