pw_kvs: Fix null access in FindSectorToGarbageCollect()

Fix a null access when logging find results in a full KVS that fails to
find a sector to garbage collect.

Change-Id: I84595d1788c393949cdbb34012d9ff1d447b1753
diff --git a/pw_kvs/key_value_store.cc b/pw_kvs/key_value_store.cc
index 9949b88..b9beed2 100644
--- a/pw_kvs/key_value_store.cc
+++ b/pw_kvs/key_value_store.cc
@@ -681,9 +681,13 @@
     }
   }
 
-  DBG("Found sector %zu to Garbage Collect, %zu recoverable bytes",
-      SectorIndex(sector_candidate),
-      RecoverableBytes(*sector_candidate));
+  if (sector_candidate != nullptr) {
+    DBG("Found sector %zu to Garbage Collect, %zu recoverable bytes",
+        SectorIndex(sector_candidate),
+        RecoverableBytes(*sector_candidate));
+  } else {
+    DBG("Unable to find sector to garbage collect!");
+  }
   return sector_candidate;
 }