gxps-page: Use gxps_value_get_double_non_negative to parse dash array values
gxps-page: Use gxps_value_get_double_non_negative to parse dash array values

Dash array values can't be negative. Spotted by Lukas Lueg.

https://bugzilla.gnome.org/show_bug.cgi?id=715023

GNOME/libgxps,GNOME/libgxps,GNOME/libgxps