Update zmq.c

Sometimes the "zval_ptr_dtor" in "recvMulti" make a segfault, but the FREE_ZVAL works. This might not be 

My System:
I am using PHP 5.4.6-1ubuntu1.1 with zmq (latest master) as _only_ Extension with libzmq 3.2.2.

# php -i | grep zmq
zmq
libzmq version => 3.2.2