method.ops is NULL, which causes method.ops->exit to crash. This
adds a check that method.ops is not NULL before dereferencing.
Fixes:
Aborting (signal 11) [/home/jprestwo/iwd/src/iwd]
++++++++ backtrace ++++++++
0 0x7f016b59cf20 in /lib/x86_64-linux-gnu/libc.so.6
1 0x432057 in resolve_exit() at /home/jprestwo/iwd/src/resolve.c:295
2 0x403b61 in iwd_modules_exit() at /home/jprestwo/iwd/src/main.c:195
3 0x7f016b57fb97 in /lib/x86_64-linux-gnu/libc.so.6
+++++++++++++++++++++++++++
The framework enables the service specific implementations
to provide its own variations for the DNS installation tasks.
The selection of the address resolution service can be done
through dns_resolve_method setting.
The module is responsible for the configuration of the address
resolution services. It will consist of the multiple service
specific plugins such as: systemd-resolved plugin, dnsmasq
plugin, etc.
