client/player: Fixes errors found by scan-build

client/player: Fixes errors found by scan-build

This fixes the following errors:

client/player.c:1406:44: warning: Dereference of null pointer
[core.NullDereference]
        reply = endpoint_select_config_reply(msg, p->data.iov_base,
                                                  ^~~~~~~~~~~~~~~~
client/player.c:1866:2: warning: 3rd function call argument is an
uninitialized value [core.CallAndMessage]
        iov_append(&cfg->caps, data, len);
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Manika Shrivastava <manika.sh@samsung.com>
Signed-off-by: Ayush Garg <ayush.garg@samsung.com>