[repacker] fix fuzzer reported stack overflow.

Fixes https://oss-fuzz.com/testcase-detail/6014493291577344.

src/graph/graph.hh

@@ -1187,6 +1187,11 @@ struct graph_t
       }
     }
 
+    for (unsigned i = 0; i < vertices_.length; i++)
+      // parents arrays must be accurate or downstream operations like cycle detection
+      // and sorting won't work correctly.
+      check_success (!vertices_[i].parents.in_error ());
+
     parents_invalid = false;
   }
 

src/hb-repacker.hh

@@ -393,6 +393,14 @@ hb_resolve_overflows (const T& packed,
     return nullptr;
   }
 
+  if (sorted_graph.in_error ())
+  {
+    // Allocations failed somewhere
+    DEBUG_MSG (SUBSET_REPACK, nullptr,
+               "Graph is in error, likely due to a memory allocation error.");
+    return nullptr;
+  }
+
   if (!hb_resolve_graph_overflows (table_tag, max_rounds, recalculate_extensions, sorted_graph))
     return nullptr;