Handle 0 in countLeadingZerosSizet.

The JS engine will sometimes do allocations of 0 bytes on startup
(see ~JSStructuredCloneWriter > extractBuffer when the buffer is empty),
and it expects the allocator to return a valid pointer in that case.

PartitionAlloc, on the other hand, doesn't handle 0-byte allocations
specially, thus 0 gets eventually passed to countLeadingZerosSizet. On
our implementation, countLeadingZerosSizet just delegates to
countLeadingZeroes{32,64}, which wasn't ready to handle 0, presumably
causing undefined behavior. This makes countLeadingZerosSizet return
sizeof(size_t) * CHAR_BIT when 0 is passed.