perf symbols: Break infinite loop on zero-filled notes in sysfs__read_build_id()

perf symbols: Break infinite loop on zero-filled notes in sysfs__read_build_id()

sysfs__read_build_id() iterates ELF note headers from sysfs files in a
while(1) loop.  If the file contains a zero-filled note header (both
n_namesz and n_descsz are 0), the code computes n = namesz + descsz = 0
and calls read(fd, bf, 0).  read() with count 0 returns 0, which
matches the expected (ssize_t)n value, so the error check passes and
the loop repeats — reading the same zero bytes and spinning forever.

This can happen with corrupted or zero-padded sysfs pseudo-files.

Add a check for n == 0 before the read, since no valid ELF note has
both name and description of zero length.

Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Fixes: f1617b40596cb341 ("perf symbols: Record the build_ids of kernel modules too")
Reviewed-by: Ian Rogers <irogers@google.com>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>