Fix crash when freeing namespaces

remove_class_from_subclasses calls st_insert, which mallocs. Malloc is not
allowed in GC. This commit replaces the st_insert with an st_update since
we know that ns_id exists in the st_table.

The following script reproduces the crash:

    require "tempfile"

    Tempfile.create do |file|
      ns = Namespace.new
      ns.require(file)
    end

1 files changed, 8 insertions, 1 deletions

diff --git a/class.c b/class.c
index 84c8668b6b..469cc5e54f 100644
--- a/class.c
+++ b/class.c
@@ -496,6 +496,13 @@ class_get_subclasses_for_ns(struct st_table *tbl, VALUE ns_id)
     return NULL;
 }
 
+static int
+remove_class_from_subclasses_replace_first_entry(st_data_t *key, st_data_t *value, st_data_t arg, int existing)
+{
+    *value = arg;
+    return ST_CONTINUE;
+}
+
 static void
 remove_class_from_subclasses(struct st_table *tbl, VALUE ns_id, VALUE klass)
 {
@@ -516,7 +523,7 @@ remove_class_from_subclasses(struct st_table *tbl, VALUE ns_id, VALUE klass)
 
             if (first_entry) {
                 if (next) {
-                    st_insert(tbl, ns_id, (st_data_t)next);
+                    st_update(tbl, ns_id, remove_class_from_subclasses_replace_first_entry, (st_data_t)next);
                 }
                 else {
                     // no subclass entries in this ns