ppp: clarify parsing of user supplied data in ppp_set_compress()

* Split big conditional statement.
  * Check (data.length <= CCP_MAX_OPTION_LENGTH) only once.
  * Don't read ccp_option[1] if not initialised.

Reading uninitialised ccp_option[1] was harmless, because this could
only happen when data.length was 0 or 1. So even then, we couldn't pass
the (ccp_option[1] < 2 || ccp_option[1] > data.length) test anyway.

Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>