pager: don't return invalid workspace rect

Make sure that get_workspace_rect does not return invalid workspace
rectangle. Looks like this functions gets called before allocation
is set on pager widget. In this case set rect->{x,y,widht,height}
to 0.

This fixes following bug in gnome-pane:
*** BUG ***
In pixman_region32_init_rect: Invalid rectangle passed
Set a breakpoint on '_pixman_log_error' to debug
WnckPager: be more restrictive on allocation check
tasklist: fix missing icon in tasklist button

After showing/hiding icon and/or label tasklist button might not
look as expected - label on icon or icon not visible at all.

https://bugzilla.gnome.org/show_bug.cgi?id=658106