bluetooth: host: conn: Check if *conn is not NULL

This commit adds a warning and a Kconfig option to `bt_conn_le_create`
and `bt_conn_le_create_synced` functions which are meant to warn a user
of a potential leakage of an active connection object.

This change is implemented due to frequent incorrect use of the
connection pointer where a pointer to an existing connection object
is overwritten by `bt_conn_le_create` and `bt_conn_le_create_synced`
functions which in turns leads to sporadic critical bugs. See
https://github.com/zephyrproject-rtos/zephyr/pull/78284#discussion_r1754304535


for more details.

The Kconfig option is introduced instead of always returning the error
to not affect current implementations. However, it is recommended to
keep this option enabled to avoid potential bugs.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>