Fix file descriptor leak when opening invalid archives.

Also add -Wunreachable-code to the set of compiler flags, otherwise
noreturn becomes considerably less useful.

bug: https://code.google.com/p/android/issues/detail?id=171099
Change-Id: I9a95d45633c731c7046d4e4a39844d9cebfd1718
(cherrypick of 241bcf05.)