vala: Handle unavailable type-symbol in Constant.check_const_type()
vala: Handle unavailable type-symbol in Constant.check_const_type()

This caused criticals like:

  vala_typesymbol_is_subtype_of: assertion 'self != NULL' failed

frida/vala,GNOME/vala,frida/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala,frida/vala,GNOME/vala