Remove useless SCOPED_MUTEX_LOCK in NetAccept::do_blocking_accept()

[masaori335]

While tracking #4726, we found a SCOPED_MUTEX_LOCK which doesn't make sense.
This might introduce other crash, but it will give us some clues of root cause.

[oknet]

@masaori335

The mutex of NetAccept::action_->continuation is optional when the EVENT_ERROR event is called back.

diff --git a/iocore/net/UnixNetAccept.cc b/iocore/net/UnixNetAccept.cc
index 21f39c4ac..9c3e40fb7 100644
--- a/iocore/net/UnixNetAccept.cc
+++ b/iocore/net/UnixNetAccept.cc
@@ -322,7 +322,7 @@ NetAccept::do_blocking_accept(EThread *t)
         return 0;
       }
       if (!action_->cancelled) {
-        SCOPED_MUTEX_LOCK(lock, action_->mutex, t);
+        SCOPED_MUTEX_LOCK(lock, action_->mutex ? action_->mutex : t->mutex, t);
         action_->continuation->handleEvent(EVENT_ERROR, (void *)(intptr_t)res);
         Warning("accept thread received fatal error: errno = %d", errno);
       }

[masaori335]

Duplicated with #4755.