Fix colorbar NaN label color leaking from MAP_FRAME_PEN

[Chuan1937]

In src/psscale.c, prefix gmt_setfont and suffix gmt_setpen around the 4 NaN text gmt_map_textcalls:

if (Ctrl->D.etext) {
    form = gmt_setfont (GMT, &GMT->current.setting.font_annot[GMT_PRIMARY]);
    gmt_map_text (GMT, ..., form);
    gmt_setpen (GMT, &GMT->current.setting.map_frame_pen);
}

Right is fixed:

Fixes #9026

[Esteban82]

I can confirm that it works.