Fix possible dereference null pointer (src/backend/replication/logical/reorderbuffer.c)

Hi,

Per Coverity.

The function ReorderBufferTXNByXid,
can return NULL when the parameter *create* is false.

In the functions ReorderBufferSetBaseSnapshot
and ReorderBufferXidHasBaseSnapshot,
the second call to ReorderBufferTXNByXid,
pass false to *create* argument.

In the function ReorderBufferSetBaseSnapshot,
fixed passing true as argument to always return
a valid ReorderBufferTXN pointer.

In the function ReorderBufferXidHasBaseSnapshot,
fixed by checking if the pointer is NULL.

best regards,
Ranier Vilela