arm/translate-a64: fix uninitialized variable warning

Fixes:
target/arm/translate-a64.c: In function 'disas_crypto_three_reg_sha512':
target/arm/translate-a64.c:13625:9: error: 'genfn' may be used uninitialized in this function [-Werror=maybe-uninitialized]
    genfn(tcg_rd_ptr, tcg_rn_ptr, tcg_rm_ptr);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
qemu/target/arm/translate-a64.c:13609:8: error: 'feature' may be used uninitialized in this function [-Werror=maybe-uninitialized]
    if (!feature) {
Reported-by: NEuler Robot <euler.robot@huawei.com>
Signed-off-by: NPan Nengyuan <pannengyuan@huawei.com>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200108023915.52288-1-pannengyuan@huawei.com>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>

target/arm/translate-a64.c

@@ -13585,6 +13585,8 @@ static void disas_crypto_three_reg_sha512(DisasContext *s, uint32_t insn)
             feature = dc_isar_feature(aa64_sha3, s);
             genfn = NULL;
             break;
+        default:
+            g_assert_not_reached();
         }
     } else {
         switch (opcode) {