220.127.116.11: fix reading of a suppressed #. in sb-cover
* Suppressing of #. in the source location recording read-table was
implemented incorrectly. (Reported by James Knight)
* Don't silently ignore read errors.
* Use a dummy source location map for top level forms for which
an error was signaled, rather than NIL. Otherwise the compiler's
and sb-cover's tlf numbering could get out of sync.