Fix pointer cast for seed calculation on 32-bit systems

Fix pointer cast for seed calculation on 32-bit systems

The fallback seed for when pg_strong_random cannot generate a high
quality seed mixes in the address of the conn object, but the cast
failed to take the word size into consideration. Fix by casting to
a uintptr_t instead. The seed calculation was added in 7f5b19817e.

The code as it stood generated the following warning on mamba and
lapwing in the buildfarm:

fe-connect.c: In function 'libpq_prng_init':
fe-connect.c:1048:11: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
1048 |  rseed = ((uint64) conn) ^
     |           ^

Author: Hayato Kuroda <kuroda.hayato@fujitsu.com>
Discussion: https://postgr.es/m/TYAPR01MB58665250EDCD551CCA9AD117F58E9@TYAPR01MB5866.jpnprd01.prod.outlook.com