gdb: Fix incorrect variable name in scoped_switch_fork_info

The previous commit:

  commit 1ef8573cc7
  Date:   Wed Jan 9 14:02:39 2019 +0000

      gdb: Improve scoped_switch_fork_info class

contained a bug, an incorrect variable name was used. Fixed in this
commit.

gdb/ChangeLog:

	* linux-fork.c (scoped_switch_fork_info)
	<~scoped_switch_fork_info>: Fix incorrect variable name.

gdb/ChangeLog

@@ -1,3 +1,8 @@
+2019-01-10  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* linux-fork.c (scoped_switch_fork_info)
+	<~scoped_switch_fork_info>: Fix incorrect variable name.
+
 2019-01-10  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* linux-fork.c (scoped_switch_fork_info)

gdb/linux-fork.c

@@ -481,7 +481,7 @@ public:
 	CATCH (ex, RETURN_MASK_ALL)
 	  {
 	    warning (_("Couldn't restore checkpoint state in %s: %s"),
-		     target_pid_to_str (fp->ptid), ex.message);
+		     target_pid_to_str (m_oldfp->ptid), ex.message);
 	  }
 	END_CATCH
       }