Fix memory leaks

This CL fixes leaks of the policy that we're building up. The analyzer
only caught the leaks on the error path, but I assume that
`check_assertions` does nothing to free the object that it's handed.

Analyzer warnings:

system/sepolicy/tools/sepolicy-analyze/neverallow.c:439:9: warning:
Potential leak of memory pointed to by 'avrule'
[clang-analyzer-unix.Malloc]

system/sepolicy/tools/sepolicy-analyze/neverallow.c:439:9: warning:
Potential leak of memory pointed to by 'neverallows'
[clang-analyzer-unix.Malloc]

Bug: None
Test: Treehugger; reran the analyzer
Change-Id: I79a0c34e8b53d33a1f01497337590eab660ad3ec