Currently smap_get_int/ullong doesn't check any conversion errors.
Most implementations of atoi/strtoull return 0 in case of failure.

This leads to returning zero in case of wrongly set database values.
For example, commands

	ovs-vsctl set interface iface options:key=\"\"
	ovs-vsctl set interface iface options:key=qwe123
	ovs-vsctl set interface iface options:key=abc

will have exactly same effect as

	ovs-vsctl set interface iface options:key=0

in case where 'key' is an integer option of the iface.
Can be checked with 'other_config:emc-insert-inv-prob' or other
integer 'options' and 'other_config's.

0 could be not a default and not safe value for many options and
it'll be better to return default value instead if any.

Conversion functions from 'util' library used to provide proper
error handling.

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Tested-by: Jan Scheurich <jan.scheurich@ericsson.com>
Acked-by: Jan Scheurich <jan.scheurich@ericsson.com>