base-stats: Fix null addStatGroup

A group must be provided to be added to a stat
group.

Change-Id: I9da42fb12c2a8b258f9f45922a6fb6b7fd41a698
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/43012
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>

src/base/stats/group.cc

@@ -113,6 +113,7 @@ Group::addStat(statistics::Info *info)
 void
 Group::addStatGroup(const char *name, Group *block)
 {
+    panic_if(!block, "Can't add null stat group %s", name);
     panic_if(block == this, "Stat group can't be added to itself");
     panic_if(statGroups.find(name) != statGroups.end(),
              "Stats of the same group share the same name `%s`.\n", name);