aarch64: don't try resuming if target is not halted

At framework level, the resume hook is not protected. Make sure to
not attempt a resume if the target is not halted.

Change-Id: I4dd1975a95d6c513bd4f4e999e496bc11182a97a
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>

src/target/aarch64.c

@@ -624,6 +624,10 @@ static int aarch64_resume(struct target *target, int current,
 		target_call_event_callbacks(target, TARGET_EVENT_RESUMED);
 		return 0;
 	}
+
+	if (target->state != TARGET_HALTED)
+		return ERROR_TARGET_NOT_HALTED;
+
 	aarch64_internal_restore(target, current, &addr, handle_breakpoints,
 				 debug_execution);
 	if (target->smp) {