target/s390x: Fix CSST for 16-byte store

Found by Coverity (CID 1378273).
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Reported-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NRichard Henderson <rth@twiddle.net>

target/s390x/mem_helper.c

@@ -1580,6 +1580,7 @@ uint32_t HELPER(csst)(CPUS390XState *env, uint32_t r3, uint64_t a1, uint64_t a2)
                 cpu_stq_data_ra(env, a2 + 0, svh, ra);
                 cpu_stq_data_ra(env, a2 + 8, svl, ra);
             }
+            break;
         default:
             g_assert_not_reached();
         }