translate-all: fix uninitialized tb->orig_tb

translate-all: fix uninitialized tb->orig_tb

This fixes a segmentation fault in icount mode when executing
from an IO region.

TB is marked as CF_NOCACHE but tb->orig_tb is not initialized
(equals previous value in code_gen_buffer).

The issue happens in cpu_io_recompile() when it tries to invalidate orig_tb.

Reviewed-by: Richard Henderson <[email protected]>
Signed-off-by: Clement Deschamps <[email protected]>
Message-Id: <20191022140016[email protected]>
Signed-off-by: Richard Henderson <[email protected]>