Avoid read of uninitialized values - detected with valgrind:

==30681== Conditional jump or move depends on uninitialised value(s)
==30681==    at 0xD1732EB: pg_tmbc_alloc_query_params (pg_type_map_by_column.c:157)
==30681==    by 0x4EA93B6: rb_protect (eval.c:819)
==30681==    by 0xD1770DE: alloc_query_params (pg_connection.c:1030)
==30681==    by 0xD17A357: pgconn_exec_params (pg_connection.c:1102)
==30681==    by 0x4FCE439: vm_call_cfunc_with_frame (vm_insnhelper.c:1470)
==30681==    by 0x4FDA48E: vm_call_method (vm_insnhelper.c:1560)
==30681==    by 0x4FD2568: vm_exec_core (insns.def:1028)
==30681==    by 0x4FD7126: vm_exec (vm.c:1304)
==30681==    by 0x4FD8EFA: invoke_block_from_c.part.49 (vm.c:732)
==30681==    by 0x4FDCB13: yield_under (vm.c:704)
==30681==    by 0x4FCE439: vm_call_cfunc_with_frame (vm_insnhelper.c:1470)
==30681==    by 0x4FDA48E: vm_call_method (vm_insnhelper.c:1560)