Changes variables destructor to virtual

Avoid memory leak while destroying the Variable objects.

src/variables/variable.h

@@ -64,6 +64,7 @@ class Variable {
 
     explicit Variable(std::string _name);
     Variable(std::string name, VariableKind kind);
+    virtual ~Variable() { }
 
     static std::string to_s(std::vector<Variable *> *variables);
 

src/variables/variations/count.h

@@ -34,6 +34,7 @@ class Count : public Variable {
     explicit Count(Variable *v)
         : Variable("count(" + v->m_name + ")"),
         var(v) { }
+    virtual ~Count() { delete var; }
 
     void evaluateInternal(Transaction *transaction,
         std::vector<const collection::Variable *> *l) override;

src/variables/variations/exclusion.h

@@ -37,6 +37,8 @@ class Exclusion : public Variable {
         var(v)
         { m_isExclusion = true; }
 
+    virtual ~Exclusion() { delete var; }
+
     void evaluateInternal(Transaction *transaction,
         std::vector<const collection::Variable *> *l) override;