[Tests] Prevent warning: variable 'x' is uninitialized #10272

#9792 brought a few "uninitialized" warnings:

bench/crypto_hash.cpp:79:13: warning: variable 'x' is uninitialized when used here [-Wuninitialized]
            x += rng.rand32();
            ^
bench/crypto_hash.cpp:76:15: note: initialize the variable 'x' to silence this warning
    uint32_t x;
              ^
               = 0
bench/crypto_hash.cpp:90:13: warning: variable 'x' is uninitialized when used here [-Wuninitialized]
            x += rng.randbool();
            ^
bench/crypto_hash.cpp:87:15: note: initialize the variable 'x' to silence this warning
    uint32_t x;
              ^
               = 0
2 warnings generated.

Fix them.

Trivial ACK (luckily is just bench code).

utACK 5ec8836

utACK 5ec883667fe6fe323fe3ae42ee54cff77450da7d