Fix -Wreturn-type warnings

When calling assert(0) instead of returning a value, -Wreturn-type
warnings are triggered because assertions can be disabled. Replace these
assertions with abort().

types/tablet_v2/wlr_tablet_v2_tool.c

@@ -75,10 +75,8 @@ static enum zwp_tablet_tool_v2_type tablet_type_from_wlr_type(
 		return ZWP_TABLET_TOOL_V2_TYPE_LENS;
 	default:
 		/* We skip these devices earlier on */
-		assert(false && "Unreachable");
+		abort(); // unreachable
 	}
-
-	assert(false && "Unreachable");
 }
 
 void destroy_tablet_tool_v2(struct wl_resource *resource) {

types/xdg_shell/wlr_xdg_positioner.c

@@ -176,7 +176,7 @@ static bool positioner_anchor_has_edge(enum xdg_positioner_anchor anchor,
 			anchor == XDG_POSITIONER_ANCHOR_TOP_RIGHT ||
 			anchor == XDG_POSITIONER_ANCHOR_BOTTOM_RIGHT;
 	default:
-		assert(false); // not reached
+		abort(); // unreachable
 	}
 }