target/riscv: Quiet Coverity complains about vamo*

Signed-off-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20200721133742.2298-1-zhiwei_liu@c-sky.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/insn_trans/trans_rvv.inc.c

@@ -733,6 +733,7 @@ static bool amo_op(DisasContext *s, arg_rwdvm *a, uint8_t seq)
             g_assert_not_reached();
 #endif
         } else {
+            assert(seq < ARRAY_SIZE(fnsw));
             fn = fnsw[seq];
         }
     }