Fix crash if SIXEL colour register is invalid and remove SIXEL images before

reflow to avoid a different crash, from Anindya Mukherjee.

image-sixel.c

@@ -489,6 +489,9 @@ sixel_print(struct sixel_image *si, struct sixel_image *map, size_t *size)
 		colours = si->colours;
 		ncolours = si->ncolours;
 	}
+
+	if (ncolours == 0)
+		return (NULL);
 	contains = xcalloc(1, ncolours);
 
 	len = 8192;

input.c

@@ -1839,7 +1839,7 @@ input_csi_dispatch_sm_private(struct input_ctx *ictx)
 
 /* Handle CSI graphics SM. */
 static void
-input_csi_dispatch_sm_graphics(struct input_ctx *ictx)
+input_csi_dispatch_sm_graphics(__unused struct input_ctx *ictx)
 {
 #ifdef ENABLE_SIXEL
 	int	n, m, o;

screen-write.c

@@ -2283,6 +2283,10 @@ screen_write_sixelimage(struct screen_write_ctx *ctx, struct sixel_image *si,
 		new = sixel_scale(si, 0, 0, 0, y - sy, sx, sy, 1);
 		sixel_free(si);
 		si = new;
+
+		/* Bail out if the image cannot be scaled. */
+		if (si == NULL)
+			return;
 		sixel_size_in_cells(si, &x, &y);
 	}
 

screen.c

@@ -308,12 +308,12 @@ screen_resize_cursor(struct screen *s, u_int sx, u_int sy, int reflow,
 	if (sy != screen_size_y(s))
 		screen_resize_y(s, sy, eat_empty, &cy);
 
-	if (reflow) {
 #ifdef ENABLE_SIXEL
-		image_free_all(s);
+	image_free_all(s);
 #endif
+
+	if (reflow)
 		screen_reflow(s, sx, &cx, &cy, cursor);
-	}
 
 	if (cy >= s->grid->hsize) {
 		s->cx = cx;