net: sctp: Move sequence start handling into sctp_transport_get_idx()

net/sctp/proc.c: In function ‘sctp_transport_get_idx’:
net/sctp/proc.c:313: warning: ‘obj’ may be used uninitialized in this function

This is currently a false positive, as all callers check for a zero
offset first, and handle this case in the exact same way.

Move the check and handling into sctp_transport_get_idx() to kill the
compiler warning, and avoid future bugs.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>