MemoryBuffer.cpp: Don't peek the next page if file is multiple of *physical*...

MemoryBuffer.cpp: Don't peek the next page if file is multiple of *physical* pagesize(4k) but is not multiple of AllocationGranularity(64k), when a null terminator is required, on cygwin and win32.

For example, r189780's SparcISelLowering.cpp has the size 98304. It crashed clang to touch a null terminator on cygwin.

FIXME: It's not good to hardcode 4096 here. dwPageSize shows 4096.
llvm-svn: 189939