samples: net: hrpl_border_router: Fix NULL pointer dereference

Null-checking `pkt` before making a call to net_pkt_unref() suggests
that it might be NULL, but the pointer is dereferenced before it's
checked.  Perform the check at the beginning of the function and
dereference and drop its reference unconditionally afterwards.

Coverity-ID: 178789
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>