Minor optimization and cleanup for PositionCodec

Summary:
Both getVarint32Ptr and getVarint32PtrFallback return nullptr when
p >= limit. Avoid the call to getVarint32PtrFallback and immediately
return from getVarint32.

As it is safe to call getVarint32 with p >= limit, skip the extra check in
each getDifferentialVarInt32 loop iteration, speeds up decoding results
for the 3 benchmark cases by ~10%.

Remove unnecessary reinterpret_cast<char*>(p), p is a char*.

Test Plan:
positioncodecbenchmark
ctest

Reviewers: Baloo, Frameworks, ngraham, astippich, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D17332