target-i386: Fix addr16 prefix

While ADDSEG will only be false in 16-bit mode for LEA, it can be
false even in other cases when 16-bit addresses are obtained via
the 67h prefix in 32-bit mode.  In this case, gen_lea_v_seg forgets
to add a nonzero FS or GS base if CS/DS/ES/SS are all zero.  This
case is pretty rare but happens when booting Windows 95/98, and
this patch fixes it.

The bug is visible since commit d6a29149, but it was introduced
together with gen_lea_v_seg and it probably could be reproduced
with a "addr16 gs movsb" instruction as early as in commit
ca2f29f5.
Reported-by: NHervé Poussineau <hpoussin@reactos.org>
Tested-by: NHervé Poussineau <hpoussin@reactos.org>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Message-Id: <1456931078-21635-1-git-send-email-pbonzini@redhat.com>
Signed-off-by: NRichard Henderson <rth@twiddle.net>

target-i386/translate.c

@@ -466,15 +466,15 @@ static void gen_lea_v_seg(DisasContext *s, TCGMemOp aflag, TCGv a0,
         break;
     case MO_16:
         /* 16 bit address */
-        if (ovr_seg < 0) {
-            ovr_seg = def_seg;
-        }
         tcg_gen_ext16u_tl(cpu_A0, a0);
-        /* ADDSEG will only be false in 16-bit mode for LEA.  */
-        if (!s->addseg) {
-            return;
-        }
         a0 = cpu_A0;
+        if (ovr_seg < 0) {
+            if (s->addseg) {
+                ovr_seg = def_seg;
+            } else {
+                return;
+            }
+        }
         break;
     default:
         tcg_abort();