Fix a stack buffer overflow with mbedtls_net_poll() and
mbedtls_net_recv_timeout() when given a file descriptor that is beyond
FD_SETSIZE. The bug was due to not checking that the file descriptor
is within the range of an fd_set object.
Fix#4169
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>