Fix memory leak when copying ST tables

st_copy allocates a st_table, which is not needed for hashes since it is
allocated by VWA and embedded, so this causes a memory leak.

The following script demonstrates the issue:

```ruby
20.times do
  100_000.times do
    {a: 1, b: 2, c: 3, d: 4, e: 5, f: 6, g: 7, h: 8, i: 9}
  end

  puts `ps -o rss= -p #{$$}`
end
```

1 files changed, 23 insertions, 11 deletions

diff --git a/st.c b/st.c
index 7d44171e97..119a6964b4 100644
--- a/st.c
+++ b/st.c
@@ -1228,17 +1228,10 @@ st_insert2(st_table *tab, st_data_t key, st_data_t value,
     return 1;
 }
 
-/* Create and return a copy of table OLD_TAB.  */
+/* Create a copy of old_tab into new_tab. */
 st_table *
-st_copy(st_table *old_tab)
+st_replace(st_table *new_tab, st_table *old_tab)
 {
-    st_table *new_tab;
-
-    new_tab = (st_table *) malloc(sizeof(st_table));
-#ifndef RUBY
-    if (new_tab == NULL)
-        return NULL;
-#endif
     *new_tab = *old_tab;
     if (old_tab->bins == NULL)
         new_tab->bins = NULL;
@@ -1246,7 +1239,6 @@ st_copy(st_table *old_tab)
         new_tab->bins = (st_index_t *) malloc(bins_size(old_tab));
 #ifndef RUBY
         if (new_tab->bins == NULL) {
-            free(new_tab);
             return NULL;
         }
 #endif
@@ -1255,7 +1247,6 @@ st_copy(st_table *old_tab)
                                                  * sizeof(st_table_entry));
 #ifndef RUBY
     if (new_tab->entries == NULL) {
-        st_free_table(new_tab);
         return NULL;
     }
 #endif
@@ -1263,6 +1254,27 @@ st_copy(st_table *old_tab)
            get_allocated_entries(old_tab));
     if (old_tab->bins != NULL)
         MEMCPY(new_tab->bins, old_tab->bins, char, bins_size(old_tab));
+
+    return new_tab;
+}
+
+/* Create and return a copy of table OLD_TAB.  */
+st_table *
+st_copy(st_table *old_tab)
+{
+    st_table *new_tab;
+
+    new_tab = (st_table *) malloc(sizeof(st_table));
+#ifndef RUBY
+    if (new_tab == NULL)
+        return NULL;
+#endif
+
+    if (st_replace(new_tab, old_tab) == NULL) {
+        st_free_table(new_tab);
+        return NULL;
+    }
+
     return new_tab;
 }