Fix T44644 Missing thread-protection (spinlock) of image in new multiview code.

Many thanks to Sergey, for practically giving the solution!

Note that it may also fix T44345, depends whether there are other missing
protections/locks or not...

source/blender/blenkernel/intern/image.c

@@ -2427,11 +2427,15 @@ void BKE_image_verify_viewer_views(const RenderData *rd, Image *ima, ImageUser *
 	}
 
 	if (do_reset) {
+		BLI_spin_lock(&image_spin);
+
 		image_free_cached_frames(ima);
 		BKE_image_free_views(ima);
 
 		/* add new views */
 		image_viewer_create_views(rd, ima);
+
+		BLI_spin_unlock(&image_spin);
 	}
 
 	BLI_unlock_thread(LOCK_DRAW_IMAGE);