vfscanf-internal: Remove potentially unbounded allocas

Some locales define a list of mapping pairs of alternate digits and
separators for input digits (to_inpunct).  This require the scanf
to create a list of all possible inputs for the optional type
modifier 'I'.

Checked on x86_64-linux-gnu.

Reviewed-by: Joe Simmons-Talbott <josimmon@redhat.com>