arm/translate-a64: fix uninitialized variable warning

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: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Pan Nengyuan <pannengyuan@huawei.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200108023915.52288-1-pannengyuan@huawei.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>