In gimp_page_selector_add_frame(), if "gimp-frame" icon cannot be loaded
(which should not happen, but reality can always strike back!), we want
to abort from gimp_page_selector_add_frame() immediately.

Also as a consequence, its return value might be NULL, hence should be
freed with g_clear_object() instead.

This happened here because of broken meson rules (which didn't install
this icon) and ended up in forever looping errors when loading a
multi-page PDF (pages are shown in frames in a dialog):

> (file-pdf-load:12348): GdkPixbuf-CRITICAL **: 11:59:28.513: gdk_pixbuf_copy_area: assertion 'src_pixbuf != NULL' failed