alsdec: validate time diff index

If begin is smaller than t, the subtraction 'begin -= t' wraps around,
because begin is unsigned. The same applies for end < t.

This causes segmentation faults.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
