fix uninitialized m_companionId in btMultiBody (only happens if sleeping is enabled)

src/BulletDynamics/Featherstone/btMultiBody.cpp

@@ -112,6 +112,7 @@ btMultiBody::btMultiBody(int n_links,
 		m_userObjectPointer(0),
 		m_userIndex2(-1),
 		m_userIndex(-1),
+		m_companionId(-1),
 		m_linearDamping(0.04f),
 		m_angularDamping(0.04f),
 		m_useGyroTerm(true),

src/BulletDynamics/Featherstone/btMultiBodyConstraintSolver.cpp

@@ -1664,4 +1664,4 @@ void  btMultiBodyConstraintSolver::solveMultiBodyGroup(btCollisionObject** bodie
 	m_tmpNumMultiBodyConstraints = 0;
 	
 
-}
+}