Fix: add in-class default for robust_function_ after removing from init-list

Previously removed robust_function_ from constructor init-list along with
nu_end_ratio_, but robust_function_ had no in-class initializer, leading
to default-initialization of an enum class (indeterminate value). Add
explicit in-class default to WELSCH to preserve correct behavior.

Author: yaoyx689

registration/include/pcl/registration/fricp.h

@@ -177,7 +177,7 @@ class FastRobustIterativeClosestPoint
   Matrix4d
   matrixLog(const Matrix4d& transform) const;
 
-  RobustFunction robust_function_;
+  RobustFunction robust_function_ = RobustFunction::WELSCH;
   bool use_anderson_ = false;
   std::size_t anderson_history_ = 5;
   double nu_begin_ratio_ = 3.0;