configurator: avoid leaks that LeakSanitizer doesn't like

These leaks aren't really an issue since they are completely bounded,
but if one is building with leak sanitizer enabled (as
-fsanitize=address does in gcc-5.1), it kills the configurator, which
isn't very useful for us. Add the few free() calls it's looking for.

This is not an actual code issue, they just workaround
some optional compiler peculiarities.

Signed-off-by: Cody P Schafer <dev@codyps.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (split off leak change)