Previously, we'd be very strict and disallow zero-length `wiggle_runtime::Region`s
altogether (we'd actually panic which is even worse). However, we
should allow this noting that any zero-length `Region` never
overlaps since its length is, well, zero. Additionally, this makes
`path_readlink` with zero buffers cleaner and possible without
additional checks/hacks around the passed in `GuestPtr<'_, [u8]>`
buffer.
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