Fixes a leak found by ASan in alpha-blending-test.
Direct leak of 160 byte(s) in 2 object(s) allocated from:
#0 0x7f511fe11518 in calloc (/lib/x86_64-linux-gnu/libasan.so.5+0xe9518)
#1 0x7f511fc76373 in zalloc ../../git/wayland/src/wayland-private.h:232
#2 0x7f511fc76373 in proxy_create ../../git/wayland/src/wayland-client.c:422
#3 0x7f511fc79dcc in create_outgoing_proxy ../../git/wayland/src/wayland-client.c:651
#4 0x7f511fc79dcc in wl_proxy_marshal_array_constructor_versioned ../../git/wayland/src/wayland-client.c:736
#5 0x7f511fc7b17b in wl_proxy_marshal_constructor_versioned ../../git/wayland/src/wayland-client.c:873
#6 0x5583e5348f43 in wl_registry_bind /home/pq/local/include/wayland-client-protocol.h:1165
#7 0x5583e534cfbe in handle_global ../../git/weston/tests/weston-test-client-helper.c:800
#8 0x7f511f34b8ed in ffi_call_unix64 (/lib/x86_64-linux-gnu/libffi.so.6+0x68ed)
Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>