erts: Fix sigq deferred_save when inner queue is empty

If the inner queue is empty, the deferred_save would incorrectly
trigger and set the message queue pointer to the wrong place.
This commit makes it so that the message queue optimization does
not trigger when the inner queue is empty. This is a correct
solution, but not ideal as it means that the message queue optimization
will not help when you have a huge middle queue but en empty
inner queue.

erts/emulator/beam/erl_message.h

@@ -331,6 +331,8 @@ typedef struct erl_trace_message_queue__ {
         if ((P)->sig_qs.saved_last) {                                   \
             if ((P)->flags & F_DEFERRED_SAVED_LAST) {                   \
                 /* Points to middle queue; use end of inner */          \
+                /* This is later used by erts_proc_sig_handle_incoming */\
+                /* to set the save to the correct place */              \
                 (P)->sig_qs.save = (P)->sig_qs.last;                    \
                 ASSERT(!PEEK_MESSAGE((P)));                             \
             }                                                           \

erts/emulator/beam/erl_proc_sig_queue.c

@@ -3417,7 +3417,14 @@ stop: {
                 deferred_saved_last = deferred_save = 0;
             }
             else {
-                if (c_p->sig_qs.save == c_p->sig_qs.last)
+                if (c_p->sig_qs.save == c_p->sig_qs.last &&
+                    c_p->sig_qs.save != &c_p->sig_qs.first)
+                    /* When save is set to last AND DEFERRED_SAVED_LAST is
+                       set we know that we have done a ERTS_RECV_MARK_SET
+                       to the last in order to trigger a clean of the middle
+                       queue. However, we cannot know this when there
+                       are no messages in the inner queue, so in that
+                       case we have to parse the entire queue again */
                     deferred_save = !0;
                 else
                     deferred_save = 0;