Replace asprintf with snprintf in kerberospw.c

[traveltissues]

asprintf is a gnu extension function. This commit replaces calls to this
with a call to the standard snprintf function when reporting errors in
the change_user_krb5pwd function.

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[traveltissues]

@dreness I've removed the ws change (https://github.com/apple/ccs-pykerberos/compare/a4248ad387f3090fd459d3d1c702a884ef9028fa..9147b64947461152a68729390feffd5f6cecd03f). This change would be useful downstream for some packages relying on pykerberos on non-linux, thanks.