bootchart: oom-check correct variable

Coverity warned that we have already dereferenced ps->sample before
null-checking it. I suspect that's not really the issue and that
the check is checking the wrong variable.
Likely the oom-check should be on the just allocated ps->sample->next.

Found by coverity. Fixes: CID#1237765

src/bootchart/store.c

@@ -399,7 +399,7 @@ schedstat_next:
                         continue;
 
                 ps->sample->next = new0(struct ps_sched_struct, 1);
-                if (!ps->sample) {
+                if (!ps->sample->next) {
                         log_oom();
                         exit(EXIT_FAILURE);
                 }