Restore errors produced on access to NIL-ARRAYs.

Transform things into DATA-NIL-VECTOR-REF instead of DATA-VECTOR-REF,
otherwise it's flushed due to unsafely-flushable and causes many
different problems.

Fixes lp#1311421.
DISASSEMBLE: don't print unwanted stuff into *debug-io*.

Also don't error if there's no source file to be found.

Fixes lp#1251861.