The fundamental problem with the original code is that it accesses
p[-2] which is one before the beginning of the input buffer for
empty lines. rev.1.6 just moved the problem from failures when
p[-2] happens to be '\\' to failures when it happens to be '\0'.
rev.1.5 was confused about the trailing newline and other things.
I went back to rev.1.5 and fixed it. The result is the same as
Keith Bostic's final version in PR 1356 except it loses more
gracefully for excessively long input lines.
newline must be part of the pattern space i.e. `echo a|sed -e P' must print
a
a
and not
aa
This is consistent with gnu sed, SunOS, Ultrix (and probably others!)
