CErc20Delegator pads return value bytes

[jflatow]

When upgrading the cToken contracts between cDAI and cUSDT, there was an attempt to shave some gas off the delegator contract, by inlining some assembly to perform delegate calls on the delegatee contract. However, the assembly inadvertently appends extra bytes to the ABI-encoded return value, which still decode properly in the EVM and other tools, but can lead to apparent issues for systems which use less robust ABI-decoding logic.

There would be no harm in switching back to the cDAI version of the CErc20Delegator contract. The extra bytes could also be trimmed with more assembly, though this also introduces new (obscure) code to be audited.