vasnprintf: silence mingw compiler warning

vasnprintf: silence mingw compiler warning

On mingw, vasnprintf is able to use snprintf, but not its return
value; this led to a situation where gcc warns:

vasnprintf.c: In function 'vasnprintf':
vasnprintf.c:4609:21: error: variable 'has_width' set but not used [-Werror=unused-but-set-variable]

Rather than hacking through a bunch of #ifdefs, where some used both
variables and others only used one, I changed the lone use of just
'width' to also use 'has_width'.

* lib/vasnprintf.c (VASNPRINTF): Avoid unused variable warning.

Signed-off-by: Eric Blake <eblake@redhat.com>