Fix memory leak on end() -> begin() -> end()

[facchinm]

Should fix #243 and maybe some other memory related issues.

Only tested in Peripheral mode, with services and characteristics allocated:

• as global objects (arduino way)
• in function stack
• in heap

On mbed-enabled platforms, the memory check can be activated via

  ....
  mbed_stats_heap_t heap_stats;
  mbed_stats_heap_get(&heap_stats);
  Serial.print("Heap size: ");
  Serial.print(heap_stats.current_size);
  Serial.print(" / ");
  Serial.println(heap_stats.reserved_size);
  ....