Leaks in idevicemobilebackup2 and lockdown.c

This pull request contains various fixes for memory leaks in idevicemobilebackup2 and lockdown.c.

Several of the leaks occur when sending an empty plist dict to `mobilebackup2_send_status_response()`. The patch here changes each occurrence from one line to three lines, and would be cleaner if wrapped in a function explicitly for sending an OK status, but I didn't want alter the API without discussing it.