target-s390: Fix SRNMT

Fallthough into abort = oops.

Cc: qemu-trivial@nongnu.org
Cc: Alexander Graf <agraf@suse.de>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

target-s390x/translate.c

@@ -3088,6 +3088,7 @@ static ExitStatus op_srnm(DisasContext *s, DisasOps *o)
         break;
     case 0xb9: /* SRNMT */
         pos = 4, len = 3;
+        break;
     default:
         tcg_abort();
     }