Properly free extension map memory

Updated extention_map_list references could accumulate into a significant
leak.

Previous commit 4108b7e43a updated only one of map referencing pointers.
A flow record also has a reference to the same map but it is not used
when exporting. This patch also updates master_record.map_ref which
has been freed.

Freeing export_maps might not be that important, since exporting is done
only once.