win32: do not crash on invalid utf8 conversion
win32: do not crash on invalid utf8 conversion

g_utf8_to_utf16() is not guaranteed to succeed. Check the error
and return if it failed.

https://bugzilla.gnome.org/show_bug.cgi?id=696232

Distrotech/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Distrotech/gtk,Distrotech/gtk,johne53/MB3Gtk-2,Distrotech/gtk,johne53/MB3Gtk-2,johne53/MB3Gtk-2,Distrotech/gtk