imc: reject files with unfathomable sampling rates

With huge sampling rates, the table derivation method does not converge fast
enough. While fixing it using e.g. Newton-Rhapson-like methods (the curve is
nicely convex) is possible, it is much simpler to reject these cases.

The value of 96000 was arbitrarily chosen as a realistic value, though
1000000 would still work and converge.

Fixes ticket #3868.

Suggested-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>