hw/display/artist: Avoid drawing line when nothing to display

Signed-off-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20200214001303.12873-5-f4bug@amsat.org>
Signed-off-by: NRichard Henderson <richard.henderson@linaro.org>

hw/display/artist.c

@@ -579,6 +579,9 @@ static void draw_line(ARTISTState *s, int x1, int y1, int x2, int y2,
     } else {
         dy = y1 - y2;
     }
+    if (!dx || !dy) {
+        return;
+    }
 
     c1 = false;
     if (dy > dx) {