Point2PointJoint.getImpulseClamp() returns the damping value instead

Another probable copy-and-paste error:
```
    JNIEXPORT jfloat JNICALL
    Java_com_jme3_bullet_joints_Point2PointJoint_getImpulseClamp(
        JNIEnv * env,
        jobject object,
        jlong jointId) {
```
...
```
        return joint->m_setting.m_damping;
```