Fix ravenscar-thread.c for multi-target

ravenscar-thread.c needed a change to adapt to multi-target:
ravenscar_thread_target::mourn_inferior called the mourn_inferior
method on the target beneat -- but when the target beneath was the
remote target, this resulted in the ravenscar target being deleted.

Switching the order of the calls to unpush_target and the beneath's
mourn_inferior fixes this problem.

gdb/ChangeLog
2020-01-31  Tom Tromey  <tromey@adacore.com>

	* ravenscar-thread.c (ravenscar_thread_target::mourn_inferior):
	Call beneath target's mourn_inferior after unpushing.

Change-Id: Ia80380515c403adc40505a6b3420c9cb35754370

gdb/ChangeLog

+2020-01-31  Tom Tromey  <tromey@adacore.com>
+
+	* ravenscar-thread.c (ravenscar_thread_target::mourn_inferior):
+	Call beneath target's mourn_inferior after unpushing.
+
 2020-01-31  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	PR tui/9765

gdb/ravenscar-thread.c

@@ -515,8 +515,9 @@ void
 ravenscar_thread_target::mourn_inferior ()
 {
   m_base_ptid = null_ptid;
-  beneath ()->mourn_inferior ();
+  target_ops *beneath = this->beneath ();
   unpush_target (this);
+  beneath->mourn_inferior ();
 }
 
 /* Implement the to_core_of_thread target_ops "method".  */