perf pmu: Fix pmu_id() heap underwrite on empty identifier file

perf pmu: Fix pmu_id() heap underwrite on empty identifier file

pmu_id() calls filename__read_str() then strips the trailing newline
via str[len - 1] = 0.  If the PMU identifier file is empty,
filename__read_str() succeeds with len = 0.  len - 1 underflows
size_t to SIZE_MAX, writing a null byte before the heap allocation.

Add a len == 0 check before the newline stripping.

Fixes: 51d548471510843e ("perf pmu: Add pmu_id()")
Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Cc: John Garry <john.g.garry@oracle.com>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>