stdio-common: Also reject exp char w/o significand in i18n scanf [BZ #13988]

stdio-common: Also reject exp char w/o significand in i18n scanf [BZ #13988]

Fix the handling of real 'scanf' input such as "+.e" as per BZ #13988
for the i18n case as well, complementing commit 6ecec3b616ae ("Don't
accept exp char without preceding digits in scanf float parsing"), where
the 'e' character is incorrectly consumed from input.  Add a test case
matching stdio-common/bug26.c, with bits from localedata/tst-sscanf.c.

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