; bindat (strz): Consistent length type check, take two

; bindat (strz): Consistent length type check, take two

Commit 30ec4a7347b2944818c6fc469ae871374ce7caa4 is incorrect -- the
length computation logic uses a simple nilness test, not `numberp'.
The `numberp' case is just an optimization if `len' is a literal
number; it does not affect the behavior.

Revert that commit, add some comments to help future readers avoid the
same mistake, and update the pack logic to use the same optimization
as the length computation for consistency.