Update array.hpp (#3769)

* Update array.hpp

Problem:Calling back on an empty container causes undefined behavior.
Solution:Check with the function empty()

* Create xqcool.mq

Adds relicensing grant

RELICENSE/xqcool.mq

@@ -0,0 +1,15 @@
+# Permission to Relicense under MPLv2 or any other share-alike OSI approved license chosen by the current ZeroMQ BDFL
+
+This is a statement by xqcool
+that grants permission to relicense its copyrights in the libzmq C++
+library (ZeroMQ) under the Mozilla Public License v2 (MPLv2) or any other 
+share-alike Open Source Initiative approved license chosen by the current 
+ZeroMQ BDFL (Benevolent Dictator for Life).
+
+A portion of the commits made by the Github handle "xqcool", with
+commit author "xqcool <xuquan316@live.com>", are copyright of xqcool .
+This document hereby grants the libzmq project team to relicense libzmq, 
+including all past, present and future contributions of the author listed above.
+
+xqcool 
+2020/01/01

src/array.hpp

@@ -100,8 +100,9 @@ template <typename T, int ID = 0> class array_t
 
  inline void erase (size_type index_)
  {
- if (_items.back ())
- ((item_t *) _items.back ())->set_array_index ((int) index_);
+ if (_items.empty ())
+ return;
+ ((item_t *) _items.back ())->set_array_index ((int) index_);
  _items[index_] = _items.back ();
  _items.pop_back ();
  }