64-bit PPSSPP(Windows): Attempt to fix games that crash with Atrac3plusdecoder64.dll. It seems to be caused by m_audiocontext being null..
Change nullptr to NULL to avoid possible issues with platforms that don't support C++11.
Merge pull request #2941 from thedax/audioFix

64-bit PPSSPP(Windows): Attempt to fix games that crash with atrac3+ DLLs.