Fixes for problems found by Coverity in the nuttx repository:

    net/socket/recvfrom.c: Check fromlen integrity before using it.

    net/socket/net_sockets.c: Always check for valid psock before using.

    net/tcp/tcp_send_unbuffered.c:  Avoid using psock beforing checking its integrity.

    sched/timer/timer_create.c: Fix watchdog resource leak if cannot allocate a new timer.