mbedtls_net_poll() and mbedtls_net_recv_timeout() rely on select(),
which represents sets of file descriptors through the fd_set type.
This type cannot hold file descriptors larger than FD_SETSIZE. Make
sure that these functions identify this failure code.
Without a proper range check of the file descriptor in the
mbedtls_net_xxx function, this test fails when running with UBSan:
```
net_poll beyond FD_SETSIZE ........................................ source/library/net_sockets.c:482:9: runtime error: index 16 out of bounds for type '__fd_mask [16]'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior source/library/net_sockets.c:482:9 in
```
This is a non-regression test for
https://github.com/ARMmbed/mbedtls/issues/4169 .
The implementation of this test is specific to Unix-like platforms.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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