Fix null read crash in CPhysHinge::Activate()

m_iszTravelSoundFwd, m_iszTravelSoundBack and m_pTravelSound aren't normally defined in most maps and can crash the game during a saverestore.

Author: pivotman319-owo

src/game/server/physconstraint.cpp

@@ -940,10 +940,13 @@ void CPhysHinge::Activate( void )
 	m_soundInfo.OnActivate(this);
 	if (m_pConstraint)
 	{
-		m_soundInfo.StartThinking(this, 
-			VelocitySampler::GetRelativeAngularVelocity(m_pConstraint->GetAttachedObject(), m_pConstraint->GetReferenceObject()) ,
-			m_hinge.worldAxisDirection
+		if (m_soundInfo.m_iszTravelSoundFwd != NULL_STRING || m_soundInfo.m_iszTravelSoundBack != NULL_STRING || m_soundInfo.m_pTravelSound != nullptr)
+		{
+			m_soundInfo.StartThinking(this, 
+				VelocitySampler::GetRelativeAngularVelocity(m_pConstraint->GetAttachedObject(), m_pConstraint->GetReferenceObject()) ,
+				m_hinge.worldAxisDirection
 			);
+		}
 
 		SetThink(&CPhysHinge::SoundThink);
 		SetNextThink( gpGlobals->curtime + m_soundInfo.getThinkRate() );