Use the getline API correctly: keep a single buffer as long as
possible, and let the callee handle re-allocation. Move the final
free out of the loop.

Release the head of the linked list.

Bug: 37757586
Test: ASAN_OPTIONS= SANITIZE_HOST=address mmma system/sepolicy
Change-Id: I42424acba7cd68c1b9a7a43e916a421ac3e253f7