Cleanup/simplify G.main usages.

Using G.main only to get main bled filepath, or built-in fonts, is OK.
So now using the 'valid' G_MAIN macro here instead.

source/blender/blenkernel/intern/curve.c

@@ -4959,7 +4959,7 @@ bool BKE_curve_minmax(Curve *cu, bool use_radius, float min[3], float max[3])
 	 */
 	if (is_font) {
 		nurb_lb = &temp_nurb_lb;
-		BKE_vfont_to_curve_ex(G.main, NULL, cu, FO_EDIT, nurb_lb,
+		BKE_vfont_to_curve_ex(NULL, cu, FO_EDIT, nurb_lb,
 		                      NULL, NULL, NULL, NULL);
 		use_radius = false;
 	}