ovn-nbctl: Initialize arguments to avoid compilation warnings.

ovn-nbctl: Initialize arguments to avoid compilation warnings.

Output arguments for parse_priority() and dhcp_options_get() may not be
initialized when either function returns an error.

This causes compilation warnings for GCC 6.3.x regarding use of
uninitialized variable use and null-pointer-arithmetic.

Fix this by initializing priority_p* value to 0 for priority_parse()
when an error occurs during parsing.

For dhcp_options_get() set *dhcp_opts_p = dhcp_opts regardless as
dhcp_opts will be equal to NULL when an error occurs within the function
anyhow.

Cc: Jakub Sitnicki <jkbs@redhat.com>
Fixes: 3844c85de979 ("ovn-nbctl: Don't die in dhcp_options_get()."
Fixes: bc8223df3b01 ("ovn-nbctl: Don't die in parse_priority().")
Signed-off-by: Ian Stokes <ian.stokes@intel.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Jakub Sitnicki <jkbs@redhat.com>