file_utils_test: Don't write uninitialized data
file_utils_test: Don't write uninitialized data

We don't seem to care about the contents we write in this test, but if
we don't initialize it valgrind complains.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

scylladb/seastar,dreamsxin/seastar,avikivity/seastar,dreamsxin/seastar,syuu1228/seastar,syuu1228/seastar,avikivity/seastar,cloudius-systems/seastar,cloudius-systems/seastar,cloudius-systems/seastar,dreamsxin/seastar,scylladb/seastar,scylladb/seastar,avikivity/seastar,syuu1228/seastar