tools lib api: Fix missing null termination in filename__read_int/ull()

tools lib api: Fix missing null termination in filename__read_int/ull()

filename__read_int() passes a stack buffer to read() using the full
sizeof(line) and then hands it to atoi() without null-terminating.
If a sysfs file fills the 64-byte buffer exactly, atoi() reads past
the array into uninitialized stack memory.

filename__read_ull_base() has the same issue with strtoull().

Fix both by reading sizeof(line) - 1 bytes and explicitly
null-terminating after a successful read.

Fixes: 3a351127cbc682c3 ("tools lib fs: Adopt filename__read_int from tools/perf/")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>