libsepol/cil: Clear AST node after destroying bad filecon rule

Commit fb0a4ce1 (libsepol/cil: Allow paths in filecon rules to be
passed as arguments) changed when the new AST node data would be set
to point to the new filecon struct when creating a filecon rule.
This causes cil_destroy_filecon() to be called twice on the filecon
struct if there is an error when creating the filecon rule.

If there is an error when creating a filecon rule, call
cil_clear_node() after destroying the filecon struct.

Reported-by: oss-fuzz (issue 64385)
Signed-off-by: James Carter <jwcart2@gmail.com>

libsepol/cil/src/cil_build_ast.c

@@ -4197,6 +4197,7 @@ int cil_gen_filecon(struct cil_db *db, struct cil_tree_node *parse_current, stru
 exit:
 	cil_tree_log(parse_current, CIL_ERR, "Bad filecon declaration");
 	cil_destroy_filecon(filecon);
+	cil_clear_node(ast_node);
 	return rc;
 }