Fix memory leak in ast.

If a node has children the array of children was never freed.

src/ast.c

@@ -26,6 +26,7 @@ void ast_node_free(ast_node_t *node) {
     if (node->children) {
         for (size_t i = 0; i < node->len; ++i)
             ast_node_free(node->children[i]);
+        free(node->children);
     }
 
     ast_node_free_value(node);

src/ast.h

@@ -17,7 +17,6 @@ typedef enum node_id {
     NODE_INSTRUCTION,
     NODE_OPERANDS,
     NODE_OPERAND,
-    NODE_REGISTER,
     NODE_IMMEDIATE,
     NODE_MEMORY,
     NODE_NUMBER,
@@ -28,6 +27,10 @@ typedef enum node_id {
     NODE_REGISTER_OFFSET,
     NODE_PLUS_OR_MINUS,
 
+    // Validated primitives
+    NODE_REGISTER,
+    NODE_SECTION,
+
     // Primitive nodes
     NODE_IDENTIFIER,
     NODE_DECIMAL,