perf report: Prevent segfault with --no-parent

Prevent a perf report segfault with the (non sensical) --no-parent
option

Signed-off-By: Andi Kleen <ak@linux.intel.com>
Acked-by: Namhyung <namhyung@kernel.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240130185552.150578-1-ak@linux.intel.com