misc: fix multiple NULL pointer sparse warnings

Those are fixes for the following warnings:

 pcap_mm.c:119:29: warning: Using plain integer as NULL pointer
 pcap_mm.c:141:29: warning: Using plain integer as NULL pointer
 ring.c:24:31: warning: Using plain integer as NULL pointer
 flowtop.c:1114:22: warning: Using plain integer as NULL pointer
 ifpps.c:1133:29: warning: Using plain integer as NULL pointer

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>