pillowfight.cc: initialize data blob with pattern

To avoid valgrind warnings like this

==19854== Syscall param sendmsg(msg.msg_iov[0]) points to uninitialised byte(s)
==19854==    at 0x5AD7A90: __sendmsg_nocancel (syscall-template.S:82)
==19854==    by 0x618727D: lcb_io_sendv (plugin-libevent.c:177)
==19854==    by 0x4E3577A: do_send_data (event.c:281)
==19854==    by 0x4E358B0: lcb_server_event_handler (event.c:325)
==19854== Memcheck, a memory error detector
==19854== Copyright (C) 2002-2012, and GNU GPL'd, by Julian Seward et al.
==19854== Using Valgrind-3.8.1 and LibVEX; rerun with -h for copyright info
==19854== Command: /home/avsej/code/couchbase-sdk/lcb/example/pillowfight/.libs/lt-pillowfight -t1 -Q1 -I1

Change-Id: I1a444ab568e9451c22fa19f2f5184d15c563ad88
Reviewed-on: http://review.couchbase.org/25081
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>