freetype: fallback to the system font if no font is specified in the style (fix #8960)

This fixes a potential NULL pointer dereference and also enables the marq
filter to use the user specified font.
Reported-by: Tillmann Karras <tillmann@selfnet.de>
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/text_renderer/freetype.c

@@ -2674,7 +2674,8 @@ static int RenderCommon( filter_t *p_filter, subpicture_region_t *p_region_out,
     {
         text_style_t *p_style;
         if( p_region_in->p_style )
-            p_style = CreateStyle( p_region_in->p_style->psz_fontname,
+            p_style = CreateStyle( p_region_in->p_style->psz_fontname ? p_region_in->p_style->psz_fontname
+                                                                      : p_sys->psz_fontfamily,
                                    p_region_in->p_style->i_font_size > 0 ? p_region_in->p_style->i_font_size
                                                                          : p_sys->i_font_size,
                                    (p_region_in->p_style->i_font_color & 0xffffff) |