display/bochs-display: fix memory leak

Fix memory leak in bochs_display_update().  Leaks 304 bytes per frame.

Fixes: 33ebad5405
Signed-off-by: Cameron Esfahani <dirty@apple.com>
Message-Id: <d6c26e68db134c7b0c7ce8b61596ca2e65e01e12.1576013209.git.dirty@apple.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit 0d82411d0e)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

hw/display/bochs-display.c

@@ -252,6 +252,8 @@ static void bochs_display_update(void *opaque)
             dpy_gfx_update(s->con, 0, ys,
                            mode.width, y - ys);
         }
+
+        g_free(snap);
     }
 }