Silence compiler warning.

Author: anthony-tuininga

src/oracledb/impl/thin/messages.pyx

@@ -2617,7 +2617,7 @@ cdef class TransactionSwitchMessage(Message):
         cdef:
             bytes global_transaction_id, branch_qualifier, xid_bytes
             bytes internal_name = None, external_name = None
-            uint32_t format_id
+            uint32_t format_id = 0
 
         # acquire data to send to the server
         if self.xid is not None: