Use scoped_value_mark in two more places

I found a couple of spots that could use scoped_value_mark.  One of
them is a spot that didn't consider the possibility that value_mark
can return NULL.  I tend to doubt this can be seen in this context,
but nevertheless this is safer.

Regression tested on x86-64 Fedora 36.

gdb/f-lang.c

@@ -271,8 +271,8 @@ class fortran_array_repacker_base_impl
   {
     if (inner_p)
       {
-	gdb_assert (m_mark == nullptr);
-	m_mark = value_mark ();
+	gdb_assert (!m_mark.has_value ());
+	m_mark.emplace ();
       }
   }
 
@@ -282,9 +282,8 @@ class fortran_array_repacker_base_impl
   {
     if (inner_p)
       {
-	gdb_assert (m_mark != nullptr);
-	value_free_to_mark (m_mark);
-	m_mark = nullptr;
+	gdb_assert (m_mark.has_value ());
+	m_mark.reset ();
       }
   }
 
@@ -305,9 +304,9 @@ class fortran_array_repacker_base_impl
      written.  */
   LONGEST m_dest_offset;
 
-  /* Set with a call to VALUE_MARK, and then reset after calling
-     VALUE_FREE_TO_MARK.  */
-  struct value *m_mark = nullptr;
+  /* Set and reset to handle removing intermediate values from the
+     value chain.  */
+  gdb::optional<scoped_value_mark> m_mark;
 };
 
 /* A class used by FORTRAN_VALUE_SUBARRAY when repacking Fortran array

gdb/value.c

@@ -3879,7 +3879,9 @@ value::fetch_lazy_register ()
   frame_info_ptr next_frame;
   int regnum;
   struct type *type = check_typedef (this->type ());
-  struct value *new_val = this, *mark = value_mark ();
+  struct value *new_val = this;
+
+  scoped_value_mark mark;
 
   /* Offsets are not supported here; lazy register values must
      refer to the entire register.  */
@@ -3983,10 +3985,6 @@ value::fetch_lazy_register ()
 
       frame_debug_printf ("%s", debug_file.c_str ());
     }
-
-  /* Dispose of the intermediate values.  This prevents
-     watchpoints from trying to watch the saved frame pointer.  */
-  value_free_to_mark (mark);
 }
 
 /* See value.h.  */