[subset] track which glyphs have allocated memory so we can clean up correctly.

Fixes https://oss-fuzz.com/testcase-detail/5388270411579392

src/OT/glyf/CompositeGlyph.hh

@@ -330,7 +330,10 @@ struct CompositeGlyph
     for (const auto &component : it)
     {
       /* last 4 points in deltas are phantom points and should not be included */
-      if (i >= deltas.length - 4) return false;
+      if (i >= deltas.length - 4) {
+        free (o);
+        return false;
+      }
 
       unsigned comp_len = component.get_size ();
       if (component.is_anchored ())

src/OT/glyf/SubsetGlyph.hh

@@ -18,6 +18,7 @@ struct SubsetGlyph
   Glyph source_glyph;
   hb_bytes_t dest_start;  /* region of source_glyph to copy first */
   hb_bytes_t dest_end;    /* region of source_glyph to copy second */
+  bool allocated;
 
   bool serialize (hb_serialize_context_t *c,
 		  bool use_short_loca,
@@ -60,12 +61,18 @@ struct SubsetGlyph
   bool compile_bytes_with_deltas (const hb_subset_plan_t *plan,
                                   hb_font_t *font,
                                   const glyf_accelerator_t &glyf)
-  { return source_glyph.compile_bytes_with_deltas (plan, font, glyf, dest_start, dest_end); }
+  {
+    allocated = source_glyph.compile_bytes_with_deltas (plan, font, glyf, dest_start, dest_end);
+    return allocated;
+  }
 
   void free_compiled_bytes ()
   {
-    dest_start.fini ();
-    dest_end.fini ();
+    if (likely (allocated)) {
+      allocated = false;
+      dest_start.fini ();
+      dest_end.fini ();
+    }
   }
 
   void drop_hints_bytes ()

src/OT/glyf/glyf.hh

@@ -424,7 +424,6 @@ glyf::_populate_subset_glyphs (const hb_subset_plan_t   *plan,
   unsigned num_glyphs = plan->num_output_glyphs ();
   if (!glyphs.resize (num_glyphs)) return false;
 
-  unsigned idx = 0;
   for (auto p : plan->glyph_map->iter ())
   {
     unsigned new_gid = p.second;
@@ -452,11 +451,10 @@ glyf::_populate_subset_glyphs (const hb_subset_plan_t   *plan,
       if (unlikely (!subset_glyph.compile_bytes_with_deltas (plan, font, glyf)))
       {
         // when pinned at default, only bounds are updated, thus no need to free
-        if (!plan->pinned_at_default && idx > 0)
-          _free_compiled_subset_glyphs (glyphs, idx - 1);
+        if (!plan->pinned_at_default)
+          _free_compiled_subset_glyphs (glyphs, new_gid);
         return false;
       }
-      idx++;
     }
   }
   return true;