fix uninitialized variable in btMultiBody

use btAssert to detect 0 determinant in btMatrix3x3 inverse
Remove obsolete comment in PosixSharedMemory, the Windows shared memory implementation was done over a year ago

src/BulletDynamics/Featherstone/btMultiBody.h

@@ -677,6 +677,7 @@ private:
 	btMatrix3x3 m_cachedInertiaTopRight;
 	btMatrix3x3 m_cachedInertiaLowerLeft;
 	btMatrix3x3 m_cachedInertiaLowerRight;
+	bool m_cachedInertiaValid;
 
     bool m_fixedBase;