perf symbol: Avoid an undefined behavior warning

The node (nd) may be NULL and pointer arithmetic on NULL is undefined
behavior. Move the computation of next below the NULL check on the
node.

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: James Clark <james.clark@arm.com>
Link: https://lore.kernel.org/r/20230914044233.1550195-1-irogers@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>