udp: ipv4: fix potential use after free in udp_v4_early_demux()

pskb_may_pull() can reallocate skb->head, we need to move the
initialization of iph and uh pointers after its call.

Fixes: 421b3885 ("udp: ipv4: Add udp early demux")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Shawn Bohrer <sbohrer@rgmadvisors.com>
Signed-off-by: David S. Miller <davem@davemloft.net>