stdio-common: Reject significand prefixes in scanf [BZ #12701]

stdio-common: Reject significand prefixes in scanf [BZ #12701]

Reject invalid formatted scanf real input data that is comprised of a
hexadecimal prefix, optionally preceded by a sign, and with no actual
digits following owing to the field width restriction in effect.  Such
data is a prefix of, but not a matching input sequence and it is
required by ISO C to cause a matching failure.

Currently a matching success is instead incorrectly produced along with
the conversion result of zero, with the prefix wholly consumed from
input.  Where the end of input is marked by the end-of-file condition
rather than the field width restriction in effect a matching failure is
already correctly produced.

Enable input data that causes test failures without this fix in place.

Reviewed-by: Joseph Myers <josmyers@redhat.com>