server: func_tank: prevent domain error on barrel adjust (#328)

When target is too close to the tank origin, `d2 - r2` expression may become negative
causing domain error on square root, and poisoning other fields and even other entities with NaN

dlls/func_tank.cpp

@@ -626,12 +626,14 @@ void CFuncTank::AdjustAnglesForBarrel( Vector &angles, float distance )
 		if( m_barrelPos.y )
 		{
 			r2 = m_barrelPos.y * m_barrelPos.y;
-			angles.y += ( 180.0f / M_PI_F ) * atan2( m_barrelPos.y, sqrt( d2 - r2 ) );
+			if( d2 > r2 )
+				angles.y += ( 180.0f / M_PI_F ) * atan2( m_barrelPos.y, sqrt( d2 - r2 ) );
 		}
 		if( m_barrelPos.z )
 		{
 			r2 = m_barrelPos.z * m_barrelPos.z;
-			angles.x += ( 180.0f / M_PI_F ) * atan2( -m_barrelPos.z, sqrt( d2 - r2 ) );
+			if( d2 > r2 )
+				angles.x += ( 180.0f / M_PI_F ) * atan2( -m_barrelPos.z, sqrt( d2 - r2 ) );
 		}
 	}
 }