VSIPL16 — Block decrement_count() is problematic in some cases

The effects of the decrement_count() accessor for blocks are described
as "Decrease the object’s use count. If the count becomes zero, the
block deallocates itself."

This is not always possible. Consider, for instance, this simple code:

Dense<...> block(...);
block.decrement_count();

Since block is created on the stack, it cannot deallocate itself.

Similar issues will occur with blocks created using placement-new into
existing allocated memory.

At minimum, this should be addressed by a clause stating that the
object's use-count shall not be decremented to zero (or allowed to
become decremented to zero) except for blocks that have been created
using "new".