[windows] Fix two minor bugs on Windows

1. In ProcessWindows if we fail to allocate memory, we need to return LLDB_INVALID_ADDRESS rather than 0 or nullptr as that is the invalid address that LLDB looks for
2. In RegisterContextWindows in ReadAllRegisterValues, always create a new buffer. This is what the other platforms do and data_sp is always null in all tested scenarios on Windows as well

llvm-svn: 348055