Fix SoftBody memory corruption when using invalid mesh

In case of failure the new soft body wasn't created but the previous one
wasn't destroyed, causing discrepancies with visual server updates.

modules/bullet/soft_body_bullet.cpp

@@ -117,19 +117,17 @@ void SoftBodyBullet::update_visual_server(SoftBodyVisualServerHandler *p_visual_
 }
 
 void SoftBodyBullet::set_soft_mesh(const Ref<Mesh> &p_mesh) {
-	if (p_mesh.is_null()) {
-		soft_mesh.unref();
-	} else {
-		soft_mesh = p_mesh;
-	}
+	destroy_soft_body();
+
+	soft_mesh = p_mesh;
 
 	if (soft_mesh.is_null()) {
-		destroy_soft_body();
 		return;
 	}
 
-	Array arrays = soft_mesh->surface_get_arrays(0);
 	ERR_FAIL_COND(!(soft_mesh->surface_get_format(0) & VS::ARRAY_FORMAT_INDEX));
+
+	Array arrays = soft_mesh->surface_get_arrays(0);
 	set_trimesh_body_shape(arrays[VS::ARRAY_INDEX], arrays[VS::ARRAY_VERTEX]);
 }