position_in_partition: make operator= exception-safe

The copy assignment operator of _ck can throw
after _type and _bound_weight have already been changed.
This leaves position_in_partition in an inconsistent state,
potentially leading to various weird symptoms.

The problem was witnessed by test_exception_safety_of_reads.
Specifically: in cache_flat_mutation_reader::add_to_buffer,
which requires the assignment to _lower_bound to be exception-safe.

The easy fix is to perform the only potentially-throwing step first.

Fixes #15822

Closes scylladb/scylladb#15864

mutation/position_in_partition.hh

@@ -360,14 +360,19 @@ public:
             }
         }
 
+    // Strong exception guarantees.
     position_in_partition& operator=(position_in_partition_view view) {
-        _type = view._type;
-        _bound_weight = view._bound_weight;
+        // The copy assigment to _ck can throw (because it allocates),
+        // but assignments to _type and _bound_weight can't throw.
+        // Thus, to achieve strong exception guarantees,
+        // we only need to perform the _ck assigmnent before others.
         if (view._ck) {
             _ck = *view._ck;
         } else {
             _ck.reset();
         }
+        _type = view._type;
+        _bound_weight = view._bound_weight;
         return *this;
     }