Add null check to UnSSATransformer#onUseLocal to avoid NPE crashes

Author: DoHuberGS

android/src/main/java/proguard/dexfile/ir/ts/UnSSATransformer.java

@@ -572,7 +572,10 @@ protected LiveV onAssignLocal(Local local, Value value) {
 
         @Override
         protected void onUseLocal(LiveV aValue, Local local) {
-            aValue.used = true;
+            if (aValue != null)
+            {
+                aValue.used = true;
+            }
         }
 
     }