Work around a false positive in MSVC debug runtime checker

In some MSVC versions on x64 configurations, the hashing function
triggers this failure:

Run-Time Check Failure #1 - A cast to a smaller data type has caused a
loss of data.  If this was intentional, you should mask the source of
the cast with the appropriate bitmask.

This is similar to the integer sanitizer - this code is valid C++ but
MSVC decides to warn about this nonetheless. Masking the pointer's low
32 bits fixes the issue.

Fixes #357.