Fix an invalid index in awful.rules

A rule like { focus = true } would cause debug::index::miss and
debug::newindex::miss to be emitted.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/awful/rules.lua.in

@@ -121,6 +121,8 @@ function apply(c)
             local geo = c:geometry();
             geo[property] = value
             c:geometry(geo);
+        elseif property == "focus" then
+            -- This will be handled below
         elseif type(c[property]) == "function" then
             c[property](c, value)
         else