fix: Add zero and negative byte length checks to ensure

[414owen]

Summary

ensure is a publically exported function, which has some odd behaviour when given a byte length of zero or less.

It raises an error when the value is zero, but happily returns a value when the byte length is negative.

I've switched these two behaviours around.

Current behaviour:

λ> runGet (ensure (-1)) "0123"
Right "0123"
λ> runGet (isolate 0 getWord8) "0123"
Left "too few bytes\nFrom:\tdemandInput\n\n"

New behaviour:

λ> runGet (ensure (-1)) "0123"
Left "Failed reading: Attempted to ensure negative amount of bytes\nEmpty call stack\n"
λ> runGet (ensure 0) "0123"
Right ""

[414owen]

Ugh... When one of the functions in Get.hs calls ensure 0, it ends up putting the empty bytestring...
We'd also need guards around those functions... which maybe isn't the worst idea...
ensure seems like an odd thing to expose to the user, since it doesn't update the read count, and doesn't have peek in the name...

This started out as a four line change...