SONARJAVA-1391 Rely on symbolic value on stack rather than symbol

Relying on symbols lead to trouble when symbol is unknown. It is better to rely
on the symbolic value wee already created and is available on the stack

Author: benzonico

java-squid/src/main/java/org/sonar/java/se/ExplodedGraphWalker.java

@@ -455,10 +455,11 @@ private void executeLogicalAssignement(AssignmentExpressionTree tree) {
     ExpressionTree variable = tree.variable();
     if (variable.is(Tree.Kind.IDENTIFIER)) {
       ProgramState.Pop unstack = programState.unstackValue(2);
+      SymbolicValue assignedTo = unstack.values.get(0);
       SymbolicValue value = unstack.values.get(1);
       programState = unstack.state;
       SymbolicValue symbolicValue = constraintManager.createSymbolicValue(tree);
-      symbolicValue.computedFrom(ImmutableList.of(programState.getValue(((IdentifierTree) variable).symbol()), value));
+      symbolicValue.computedFrom(ImmutableList.of(assignedTo, value));
       programState = programState.put(((IdentifierTree) variable).symbol(), symbolicValue);
       programState = programState.stackValue(symbolicValue);
     }

java-squid/src/test/files/se/Reproducer.java

@@ -112,4 +112,8 @@ void foo() {
     object2.toString(); // not accessible with null value
   }
 
+  void assignUnknownSymbol() {
+    unknown |= false;
+  }
+
 }