conversions from int to short changing the values. This applies to
B38400 and higher, since their values do not fit into a short.
However, since the wrapped values are still unique, and they only serve
as keys, there is no problem in adding a cast to silence the warnings.
This also avoids changing the ABI, which would happen if we changed
NCURSES_OSPEED to int.
Discussed with: Thomas Dickey
MFC after: 1 week
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