Fix two problems that manifested themselves on the nitpicky snakes:

Fix two problems that manifested themselves on the nitpicky snakes:
- array-set! could cause an overflow exception when reading
un-initialized garbage.  ARRAY-ALLOCATE now initializes all array
components to 0.
- array-set! could cause an underflow exception when storing a valid
double too small for a float.  It now silently stores 0.

There are other problems left (overflow when storing a large
number), etc, but...