DynamicPartitionControlAndroid: fix incorrect log.

Test: pass
Change-Id: I0621f7b535288f4ca1431298628255660adc1ac5
diff --git a/dynamic_partition_control_android.cc b/dynamic_partition_control_android.cc
index 27e117c..5e2c2a4 100644
--- a/dynamic_partition_control_android.cc
+++ b/dynamic_partition_control_android.cc
@@ -126,6 +126,7 @@
     LOG(WARNING) << "No metadata slot "
                  << BootControlInterface::SlotName(source_slot) << " in "
                  << super_device;
+    return nullptr;
   }
   LOG(INFO) << "Loaded metadata from slot "
             << BootControlInterface::SlotName(source_slot) << " in "