perf report: Prevent segfault with --no-parent

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

Signed-off-By: Andi Kleen <[e-mail]>
Acked-by: Namhyung <[e-mail]>
Signed-off-by: Namhyung Kim <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

tools/perf/util/sort.c

···
 				sort_dimension_add_dynamic_header(sd);
 		}
 
-		if (sd->entry == &sort_parent) {
+		if (sd->entry == &sort_parent && parent_pattern) {
 			int ret = regcomp(&parent_regex, parent_pattern, REG_EXTENDED);
 			if (ret) {
 				char err[BUFSIZ];