xshared: Fix parsing of empty string arg in '-c' option

xshared: Fix parsing of empty string arg in '-c' option

Calling iptables with '-c ""' resulted in a call to strchr() with an
invalid pointer as 'optarg + 1' points to past the buffer. The most
simple fix is to drop the offset: The global optstring part specifies a
single colon after 'c', so getopt() enforces a valid pointer in optarg.
If it contains a comma at first position, packet counter value parsing
will fail so all cases are covered.

Reported-by: gorbanev.es@gmail.com
Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1741
Fixes: 60a6073690a45 ("Make --set-counters (-c) accept comma separated counters")
Signed-off-by: Phil Sutter <phil@nwl.cc>