[IPCOMP]: Fetch nexthdr before ipch is destroyed

When I moved the nexthdr setting out of IPComp I accidently moved
the reading of ipch->nexthdr after the decompression.  Unfortunately
this means that we'd be reading from a stale ipch pointer which
doesn't work very well.

This patch moves the reading up so that we get the correct nexthdr
value.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>