net: lib: ptp: Fix potential NULL dereference in transport prot addr

Retreiving IP address from the networking interface assigned to
a PTP port could result in NULL dereference or undefined behaviour
when passing NULL to `memcpy()` function. Added check on addresses
returned from functions retreiving IP address from an interface.

Signed-off-by: Adam Wojasinski <awojasinski@baylibre.com>