target/arm: Avoid maybe-uninitialized warning with gcc 4.9

target/arm: Avoid maybe-uninitialized warning with gcc 4.9

GCC version 4.9.4 isn't clever enough to figure out that all
execution paths in disas_ldst() that use 'fn' will have initialized
it first, and so it warns:

/home/LiKaige/qemu/target/arm/translate-a64.c: In function ‘disas_ldst’:
/home/LiKaige/qemu/target/arm/translate-a64.c:3392:5: error: ‘fn’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
     fn(cpu_reg(s, rt), clean_addr, tcg_rs, get_mem_index(s),
     ^
/home/LiKaige/qemu/target/arm/translate-a64.c:3318:22: note: ‘fn’ was declared here
     AtomicThreeOpFn *fn;
                      ^

Make it happy by initializing the variable to NULL.

Signed-off-by: Kaige Li <likaige@loongson.cn>
Message-id: 1596110248-7366-2-git-send-email-likaige@loongson.cn
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: Clean up commit message and note which gcc version this was]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>