SF bug 482574: audioop.ratecv crashes.

SF bug 482574:  audioop.ratecv crashes.
Bugfix candidate.
A numerically naive computation of output buffer size caused crashes
and spurious MemoryErrors for reasonable arguments.
audioop_ratecv():  Avoid spurious overflow by careful reworking of the
buffer size computations, triggering MemoryError if and only if the
final buffer size can't be represented in a C int (although
PyString_FromStringAndSize may legitimately raise MemoryError even if
it does fit in a C int).  All reasonable arguments should work as
intended now, and all unreasonable arguments should be cuaght.