[repacker] Fix fuzzer memory leak.

https://oss-fuzz.com/testcase-detail/6419865171525632

src/graph/graph.hh

@@ -401,9 +401,10 @@ struct graph_t
     return vertices_[i].obj;
   }
 
-  void add_buffer (char* buffer)
+  bool add_buffer (char* buffer)
   {
     buffers.push (buffer);
+    return !buffers.in_error ();
   }
 
   /*

src/graph/gsubgpos-context.cc

@@ -52,7 +52,11 @@ unsigned gsubgpos_graph_context_t::create_node (unsigned size)
   if (!buffer)
     return -1;
 
-  add_buffer (buffer);
+  if (!add_buffer (buffer)) {
+    // Allocation did not get stored for freeing later.
+    hb_free (buffer);
+    return -1;
+  }
 
   return graph.new_node (buffer, buffer + size);
 }

src/graph/gsubgpos-context.hh

@@ -47,9 +47,9 @@ struct gsubgpos_graph_context_t
 
   HB_INTERNAL unsigned create_node (unsigned size);
 
-  void add_buffer (char* buffer)
+  bool add_buffer (char* buffer)
   {
-    graph.add_buffer (buffer);
+    return graph.add_buffer (buffer);
   }
 
  private: