udp_proxy: don't drop messages in the last flight
udp_proxy: don't drop messages in the last flight

Resending the last flight is on the todo-list, but I want to be able to test
what's already done now.

ARMmbed/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls