powerpc/imc-pmu: Add a null pointer check in update_events_in_group()

kasprintf() returns a pointer to dynamically allocated memory
which can be NULL upon failure.

Fixes: 885dcd709ba9 ("powerpc/perf: Add nest IMC PMU support")
Signed-off-by: Kunwu Chan <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://msgid.link/[e-mail]

arch/powerpc/perf/imc-pmu.c

···
 	attr_group->attrs = attrs;
 	do {
 		ev_val_str = kasprintf(GFP_KERNEL, "event=0x%x", pmu->events[i].value);
+		if (!ev_val_str)
+			continue;
 		dev_str = device_str_attr_create(pmu->events[i].name, ev_val_str);
 		if (!dev_str)
 			continue;
···
 		attrs[j++] = dev_str;
 		if (pmu->events[i].scale) {
 			ev_scale_str = kasprintf(GFP_KERNEL, "%s.scale", pmu->events[i].name);
+			if (!ev_scale_str)
+				continue;
 			dev_str = device_str_attr_create(ev_scale_str, pmu->events[i].scale);
 			if (!dev_str)
 				continue;
···
 
 		if (pmu->events[i].unit) {
 			ev_unit_str = kasprintf(GFP_KERNEL, "%s.unit", pmu->events[i].name);
+			if (!ev_unit_str)
+				continue;
 			dev_str = device_str_attr_create(ev_unit_str, pmu->events[i].unit);
 			if (!dev_str)
 				continue;