Pango plugin gv_get_ps_fontlist: postpone 'family_name' allocation

Pango plugin gv_get_ps_fontlist: postpone 'family_name' allocation

Rather than allocating this upfront and then potentially needing to free it
later, we can use a string view to postpone the allocation to when we know it is
actually needed. This works because the lifetime of the backing memory extends
until the call to `g_free(families)`. This also fixes the lack of allocation
failure checks; instead of `strdup` we now call `strview_str` that gracefully
exits on allocation failure.