Fix undersized result buffer in pset_quoted_string().

Fix undersized result buffer in pset_quoted_string().

The malloc request was 1 byte too small for the worst-case output.
This seems relatively unlikely to cause any problems in practice,
as the worst case only occurs if the input string contains no
characters other than single-quote or newline, and even then
malloc alignment padding would probably save the day.  But it's
definitely a bug.

David Rowley