Fix unpacking empty array / segmentation fault.

Program received signal SIGSEGV, Segmentation fault.
add_next_index_zval (arg=0x0, value=0x159e650) at src/service/php/php-5.4.11/Zend/zend_API.c:1406
1406            return zend_hash_next_index_insert(Z_ARRVAL_P(arg), &value, sizeof(zval *), NULL);
(gdb) bt
#0  add_next_index_zval (arg=0x0, value=0x159e650) at src/service/php/php-5.4.11/Zend/zend_API.c:1406
#1  0x00007ffff3aefa52 in msgpack_unserialize_array_item (unpack=0x7fffffff11e0, container=0x7fffffff3290, obj=0x159e650)
    at src/service/php/msgpack-php/msgpack_unpack.c:498
#2  0x00007ffff3aea769 in template_execute (ctx=0x7fffffff11e0, data=<value optimized out>, len=<value optimized out>, off=<value optimized out>)
    at src/service/php/msgpack-php/msgpack/unpack_template.h:338
#3  0x00007ffff3aeb195 in php_msgpack_unserialize (return_value=0x13328c0, str=<value optimized out>, str_len=165671)
    at src/service/php/msgpack-php/msgpack.c:267