args: fix read_all() uninitialized warning

read_all() on IntArg() gives:
.../include/click/args.hh: In function ‘void args_base_read_all(Args*, const char*, int, P, Vector<T>&) [with P = IntArg, T = short unsigned int]’:
.../include/click/args.hh:815:8: warning: ‘sx’ may be used uninitialized in this function [-Wuninitialized]

..the problem being that the compiler doesn't realize postparse() guarantees
that: _read_status => sx initialized.

Reviewed-by: rtshanks@meraki.com
Signed-off-by: Patrick Verkaik <patrick@meraki.net>