bdiff: fix malloc(0) issue in fixws()
If fixws() is called on a zero-length string, malloc(0) is called and
expected to return a pointer. Which it does on e.g. Linux. AIX returns
NULL, which it is also legal, but the malloc() is then assumed to have
failed. So ensure a valid pointer is always returned.
$ "$TESTDIR/hghave" pyflakes || exit 80
$ cd `dirname $TESTDIR`
$ pyflakes mercurial hgext 2>&1 | $TESTDIR/filterpyflakes.py
hgext/inotify/linux/__init__.py:*: 'from _inotify import *' used; unable to detect undefined names (glob)