mport bootstrap index fetch can continue after hash verification failure

mport is the MidnightBSD Package Manager. Prior to 2.7.8, the mport_fetch_bootstrap_index() function in libmport/fetch.c could return success when bootstrap index hash verification encountered a missing or invalid hash because the failure path did not preserve a fatal result. A network attacker or compromised mirror able to alter bootstrap index content or its transport path could therefore cause mport to proceed with an unverified or tampered bootstrap package index. This issue is fixed in version 2.7.8.