tools: Fix possible uninitialized variable in obexctl

On set_default_session(), if g_dbus_proxy_get_property() returns FALSE,
desc will be uninitialized. Given that this function already checks for
NULL proxy internally, it is enough to check whether it fails (and if
so, set a default prompt without destination).

Fixes this clang error:

tools/obexctl.c:439:6: error: variable 'desc' is used uninitialized
whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]