Resolving infinite loop with buffers longer than MaxArrayLength

[benmwatson]

Resolves #344

As reported in #344 , there is an infinite loop that can happen when trying to call TryGetBuffer on a very large stream. The correct response is to return false.

Specifically, if the stream was longer than 1 GB, and UseExponentialLargeBuffer was set to true, it would try to pick the next available buffer size, which is 2 GB. Because of the way pool index is calculated, there was an Int32 underflow, which always compared false to the loop exit condition, thus the infinite loop.

At first, I fixed the GetPoolIndex method to do all its math in long, but this just pushes the error further down the call stack because you can't actually allocate an array that big. Rather than add new exception conditions in multiple methods, the simplest way to resolve the issue was to just move the existing array length check to be after the rounding step in GetLargeBuffer.