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.
hg debuginstall
$ hg debuginstall
Checking encoding (ascii)...
Checking installed modules (*mercurial)... (glob)
Checking templates (*mercurial?templates)... (glob)
Checking commit editor...
Checking username...
No problems detected
hg debuginstall with no username
$ HGUSER= hg debuginstall
Checking encoding (ascii)...
Checking installed modules (*mercurial)... (glob)
Checking templates (*mercurial?templates)... (glob)
Checking commit editor...
Checking username...
no username supplied (see "hg help config")
(specify a username in your configuration file)
1 problems detected, please check your install!
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