MFC r318511

The result of right shifting a negative signed value is implementation
defined. On machines without arithmetic shift instructions, zero bits
may be shifted in from the left, giving a large positive result instead
of the desired divide-by power-of-2. Fix this by operating on the
absolute value and compensating for the possible negation later.

Reverse the order of the underflow/overflow tests and the exponential
decay calculation to avoid the possibility of an erroneous overflow
detection if p is a sufficiently small non-negative value. Also
check for negative values of prob before doing the exponential decay
to avoid another instance of of right shifting a negative value.

Tested by: Rasool Al-Saadi <ralsaadi@swin.edu.au>