[repacker] fix potential use after free in repacker.

During table splitting we iterate over the lookups map which can be modified during table splitting. This can result in a use after free in the iterator. Create a local copy of the lookup indices to avoid this.

src/hb-repacker.hh

diff --git a/src/hb-repacker.hh b/src/hb-repacker.hh
index e304225..e9cd376 100644
--- a/src/hb-repacker.hh
+++ b/src/hb-repacker.hh
@@ -79,7 +79,12 @@
   //                pass after this processing is done. Not super necessary as splits are
   //                only done where overflow is likely, so de-dup probably will get undone
   //                later anyways.
-  for (unsigned lookup_index : ext_context.lookups.keys ())
+
+  // The loop below can modify the contents of ext_context.lookups if new subtables are added
+  // to a lookup during a split. So save the initial set of lookup indices so the iteration doesn't
+  // risk access free'd memory if ext_context.lookups gets resized.
+  hb_set_t lookup_indices(ext_context.lookups.keys ());
+  for (unsigned lookup_index : lookup_indices)
   {
     graph::Lookup* lookup = ext_context.lookups.get(lookup_index);
     if (!lookup->split_subtables_if_needed (ext_context, lookup_index))