Patch from Rob Landley

Patch from Rob Landley

Fixed a memory leak in add_cmd/add_cmd_str by moving the allocation
of sed_cmd down to where it's actually first needed.

In get_address, if index_of_next_unescaped_regexp_delim ever failed, we
wouldn't notice because the return value was added to idx, which was
already guaranteed to be > 0.  (This is buried in the changes made when
I redid get_address to be based on pointer arithmetic, because all the tests
were gratuitously dereferencing with a constant zero, which wasn't obvious.)

Comment in parse_regex_delim was wrong: 's' and 'y' both call it.

The reason "sed_cmd->num_backrefs = 0;" isn't needed is that sed_cmd was
allocated with cmalloc, which zeroes memory.

Different handling of space after \ in i...

Different handling of pattern "s/a/b s/c/d"

Cool, resursive reads don't cause a crash. :)

Fixed "sed -f blah filename - < filename" since GNU sed was handling
both - and filenames on the same line.  (You can even list - more than
once, although it's immediate EOF...)