text_renderer/svg: fix uaf at template load

If token is not found in svg template, psz_template is
freed. Yet it is used in svg_GetDocument as
psz_file_template and incorectly being double freed in
Destroy().

Since the only use of psz_file_template is in svg_GetDocument
and it is performed after a null-check, setting psz_template
to null after freed should fix the issue.

Fixes: #29370 (Coverity 1398436)

modules/text_renderer/svg.c

@@ -127,6 +127,7 @@ static void svg_LoadTemplate( filter_t *p_filter )
         {
             msg_Err( p_filter, "'%s' not found in SVG template", SVG_TEMPLATE_BODY_TOKEN );
             free( psz_template );
+	    psz_template = NULL;
         }
         else *((char*)p_sys->psz_token) = 0;
     }