Fix CVE-2024-43688, buffer underflow for very large step values

Fix CVE-2024-43688, buffer underflow for very large step values

In get_number(), reject values that are so large that they are
interpreted as negative numbers.  In set_range(), step values smaller
than one or larger than the "stop" value are ignored.  This prevents
bit_nset() from being called with out-of-range values.

Bug found by Dave G. of Supernetworks.