Prevent "move next" and "move prev" commands from crashing

Fixes #1120

When the parent of a view is C_WORKSPACE and the movement direction
is either MOVE_PREV or MOVE_NEXT, the code would attempt to move the
views to the next output, but swayc_adjacent_output can't accept
non-directional movement commands and causes undefined behaviour and
a segfault.

If the code is simply skipped, we end up in an infinite loop.

Instead, we can allow containers whose parent is a C_WORKSPACE take the
path that handles MOVE_PREV and MOVE_NEXT, which behaves as you would
expect.

I'm not certain that this fix is entirely correct as the desired behaviour
of move_container is not very well defined, but it seems to work.

sway/layout.c

@@ -417,7 +417,7 @@ void move_container(swayc_t *container, enum movement_direction dir, int move_am
 		sway_log(L_DEBUG, "container:%p, parent:%p, child %p,",
 				container,parent,child);
 		if (parent->layout == layout
-			|| (layout == L_NONE && parent->type == C_CONTAINER) /* accept any layout for next/prev direction */
+			|| (layout == L_NONE && (parent->type == C_CONTAINER || parent->type == C_WORKSPACE)) /* accept any layout for next/prev direction */
 			|| (parent->layout == L_TABBED && layout == L_HORIZ)
 			|| (parent->layout == L_STACKED && layout == L_VERT)
 			|| is_auto_layout(parent->layout)) {