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.

src/pugixml.cpp

@@ -378,7 +378,7 @@ PUGI__NS_BEGIN
 
 		static PUGI__UNSIGNED_OVERFLOW unsigned int hash(const void* key)
 		{
-			unsigned int h = static_cast<unsigned int>(reinterpret_cast<uintptr_t>(key));
+			unsigned int h = static_cast<unsigned int>(reinterpret_cast<uintptr_t>(key) & 0xffffffff);
 
 			// MurmurHash3 32-bit finalizer
 			h ^= h >> 16;