xmalloc: Make xfree a two part macro/inline

This way the coverity scanner will no longer complain about dereference
before NULL checks right before xfree().

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
dissector: igmp: Fix erroneous null pointer check

We need to break if we get NULL from pkt_pull() but the actual check was
inverted. This was discovered by the coverity scanner.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>