Ensure that we clear deleted values

When throwing on an invalid value, we may end up freeing the same tree
twice.  This is generally not an issue but we need to mark the freed
memory as null to avoid a double free

Fixes https://gitlab.com/kicad/code/kicad/issues/12981

common/libeval_compiler/libeval_compiler.cpp

@@ -250,6 +250,7 @@ COMPILER::~COMPILER()
     if( m_tree )
     {
         freeTree( m_tree );
+        m_tree = nullptr;
     }
 
     // Allow explicit call to destructor
@@ -694,6 +695,7 @@ void COMPILER::freeTree( LIBEVAL::TREE_NODE *tree )
         freeTree( tree->leaf[1] );
 
     delete tree->uop;
+    tree->uop = nullptr;
 }