Fix bugs listed by clang's static analyzer

A few pedantic changes and unused variables (1-4), and genuine bugs (5,
6).

The reports with the corresponding files and lines numbers are as
follows.

1. backend/libinput/tablet_pad.c@31,44,57
"Allocator sizeof operand mismatch"
"Result of 'calloc' is converted to a pointer of type 'unsigned int',
which is incompatible with sizeof operand type 'int'"

2. types/tablet_v2/wlr_tablet_v2_pad.c@371
"Allocator sizeof operand mismatch"
"Result of 'calloc' is converted to a pointer of type 'uint32_t', which
is incompatible with sizeof operand type 'int'"

3. types/wlr_cursor.c@335
"Dead initialization"
"Value stored to 'dx'/'dy' during its initialization is never read"

4. rootston/xdg_shell.c@510
"Dead initialization"
"Value stored to 'desktop' during its initialization is never read"

5. types/tablet_v2/wlr_tablet_v2_pad.c@475
"Dereference of null pointer"
"Access to field 'strips' results in a dereference of a null pointer
(loaded from field 'current_client')"

The boolean logic was incorrect (c.f. the check in the following
function).

6. examples/idle.c@163,174,182
"Uninitialized argument value"
"1st function call argument is an uninitialized value"

If close_timeout != 0, but simulate_activity_timeout >= close_timeout,
the program would segfault at pthread_cancel(t1).

rootston/xdg_shell.c

@@ -507,8 +507,6 @@ static void decoration_handle_surface_commit(struct wl_listener *listener,
 }
 
 void handle_xdg_toplevel_decoration(struct wl_listener *listener, void *data) {
-	struct roots_desktop *desktop =
-		wl_container_of(listener, desktop, xdg_toplevel_decoration);
 	struct wlr_xdg_toplevel_decoration_v1 *wlr_decoration = data;
 
 	wlr_log(WLR_DEBUG, "new xdg toplevel decoration");