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Net: Fix off-grid dragging, including a segfault

When a drag started outside the grid (or no drag has started yet),
ensure the drag state in game_ui says so and bail out accordingly.

Previously, such a drag would manipulate the tile the last valid drag
started from, if any, else segfault.

Also, allow drags that start on-grid and then go off-grid to continue
rotating.

--- a/net.c

+++ b/net.c

@@ -2041,6 +2041,9 @@

         get_random_seed(&seed, &seedsize);

         ui->rs = random_new(seed, seedsize);

         sfree(seed);

+#ifdef USE_DRAGGING

+        ui->dragstartx = ui->dragstarty = ui->dragtilex = ui->dragtiley = -1;

+#endif

     } else {

         ui->rs = NULL;

     }

@@ -2169,12 +2172,22 @@

 	 */

 	x -= WINDOW_OFFSET + LINE_THICK;

 	y -= WINDOW_OFFSET + LINE_THICK;

-	if (x < 0 || y < 0)

-	    return nullret;

 	tx = x / TILE_SIZE;

 	ty = y / TILE_SIZE;

-	if (tx >= state->width || ty >= state->height)

+	if (x < 0 || y < 0 || tx >= state->width || ty >= state->height) {

+#ifdef USE_DRAGGING

+	    if (IS_MOUSE_DOWN(button)) {

+	        ui->dragstartx = ui->dragstarty = ui->dragtilex = ui->dragtiley = -1;

+	        return nullret;

+	    }

+	    /*

+	     * else: Despite the mouse moving off the grid, let drags and releases

+	     * continue to manipulate the tile they started from.

+	     */

+#else

 	    return nullret;

+#endif

+        }

         /* Transform from physical to game coords */

         tx = (tx + ui->org_x) % state->width;

         ty = (ty + ui->org_y) % state->height;

@@ -2212,6 +2225,9 @@

                    || button == RIGHT_DRAG

 #endif

                   ) {

+            if (ui->dragtilex < 0)

+                return nullret;

+

             /*

              * Find the new drag point and see if it necessitates a

              * rotation.

@@ -2265,7 +2281,7 @@

                    || button == RIGHT_RELEASE

 #endif

                   ) {

-            if (!ui->dragged) {

+            if (!ui->dragged && ui->dragtilex >= 0) {

                 /*

                  * There was a click but no perceptible drag:

                  * revert to single-click behaviour.