Fix overflow in ByteBlockPool, #1003

[paulirwin]

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Fix overflow (resulting in ArrayIndexOutOfBoundsException) in ByteBlockPool

Fixes #1003

Description

Thanks to @hidingmyname for the provided test. This fixes an arithmetic overflow that results in an ArrayIndexOutOfBoundsException in ByteBlockPool for fields with a large number of small tokens. In .NET, the checked block causes the overflow to throw an OverflowException, just like how the upstream Java change to use Math.addExact(x, y) throws an ArithmeticException in that case.

[NightOwl888]

I wonder, could Spatial4n/Spatial4n#1 be a similar issue as this (overflow)?

[paulirwin]

I wonder, could Spatial4n/Spatial4n#1 be a similar issue as this (overflow)?

I don't think so, since it's not an addition that overflows Int32. In that case, I think it could be an out of range character in the geohash. Without trying out the code, just from thinking through it, it seems like characters above the ASCII alphanumeric set could cause that issue, such as 'Ä' (int value 196) which would result in a calculated index of (int)'Ä' - (int)'0' == 148 which is out of range. Just a thought there, if the caller is not validating that the input is a valid geohash.