xtoptions: fill in fallback value for nvals

xtoptions: fill in fallback value for nvals

Parsing for libxt_conntrack rev 2 is done by using rev 2's option
structure, which specifies XTTYPE_PORT, and using rev 3's parser
skeleton, which uses cb->nvals. Reading cb->nvals when not using
XTTYPE_PORTRC (or any other multi-value type) is undefined behavior.

Make it defined. Since XTTYPE_NONE is the only type that can take
void, nvals logically ought to be 1.

References: http://marc.info/?l=netfilter-devel&m=131370592105298&w=2
Signed-off-by: Jan Engelhardt <jengelh@medozas.de>