Fix weak pointer leak

This patch fixes a leak with lost weak references in fontset.

https://bugzilla.gnome.org/show_bug.cgi?id=738868
Increase MAX_FREED_FONTS value

Increasing MAX_FREED_FONTS value should make fontset's cache
working normally because of long alias list and available
fonts under Windows 7.

https://bugzilla.gnome.org/show_bug.cgi?id=738868