net: ieee802154_mgmt: Fix memcpy to uninitialized pointer location

As since commit bff6a5cc, params.dst.ext_addr is no longer assigned with
the ctx->coord_ext_addr pointer, but a endianness swap is included, it
should be assigned with a proper buffer before the memcpy.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>