minui: Fix breakage in graphics_adf.

When graphics_adf was refactored in 557fa1f4, a class member was introduced
that was not initialized to zero. This meant that the first time Flip()
was called, current_surface would have a junk value and cause a bad
pointer rereference, crashing recovery.

Make sure current_surface is initialized to 0 for the first Flip().

Test: Ran recovery on a device using the ADF backend.
Change-Id: I9b8fac0a4d48cac990e5e9808a071c232de1ebfb