Fixes for undefined behavior in the snmp decoder #1345
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Instead of shifting bits off the top of the 32-bit value, we mask off the top 8 bits before shifting them away, and restore the sign bit at the end. This still results in a result that is not what was intended, as this code can not handle values greater than 2^31-1 or smaller than -2^31, but this new mechanism results in a "more correct" garbage out, with no undefined behavior.
When decoding an OID, and shifting left by 7, mask off the top 7 bits first. This still results in GIGO, but avoids undefined behavior on the way there. OIDs with values this large are not supported by this code.
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These changes fix undefined behavior in the snmp decoder when the underlying representation is of a value larger than 32-bits. The decoder fundamentally doesn't support these values, and the original behavior is GIGO. We retain the same GIGO behavior, but avoiding undefined behavior.
Fixes #1054