[poincare] VariableContext::expressionForSymbol returns a clone of its

kept value to unable any modification of its value (without calling
setExpressionForSymbol).

All contexts::expressionForSymbol return new Expression that can be tempered with.

poincare/src/variable_context.cpp

@@ -31,7 +31,7 @@ void VariableContext::setExpressionForSymbol(const Expression & expression, cons
 
 const Expression VariableContext::expressionForSymbol(const SymbolAbstract & symbol) {
   if (strcmp(symbol.name(), m_name) == 0) {
-    return m_value;
+    return m_value.clone();
   } else {
     return m_parentContext->expressionForSymbol(symbol);
   }