net: ip: net_pkt: Add pkt_cursor_advance before contiguous check

When net_pkt_skip skips the entire data in a net buffer, the cursor
still points to this buffer on data that is off by one.
Calling pkt_cursor_advance in net_pkt_is_contiguous fixes this and
moves the cursor to the next buffer.

Signed-off-by: Alexander Wachter <alexander.wachter@student.tugraz.at>