mem-cache: Fix TreePLRU num leaves error (#1075)

This PR fixes the error noted here #1073. 

Change-Id: I5d31c259ac5ee93f46f28b20eda4f58460ba8523

src/mem/cache/replacement_policies/tree_plru_rp.cc

@@ -104,7 +104,8 @@ TreePLRU::TreePLRUReplData::TreePLRUReplData(
 TreePLRU::TreePLRU(const Params &p)
   : Base(p), numLeaves(p.num_leaves), count(0), treeInstance(nullptr)
 {
-    fatal_if(numLeaves < 2, "Number of leaves must be two or greater");
+    fatal_if(numLeaves < 1,
+        "numLeaves should never be 0");
 }
 
 void
@@ -191,7 +192,7 @@ TreePLRU::getVictim(const ReplacementCandidates& candidates) const
 
     // The tree index is currently at the leaf of the victim displaced by the
     // number of non-leaf nodes
-    return candidates[tree_index - (numLeaves - 1)];
+    return candidates.at(tree_index - (numLeaves - 1));
 }
 
 std::shared_ptr<ReplacementData>