use SvCUR(PL_numeric_radix_sv) not SvLEN()

use SvCUR(PL_numeric_radix_sv) not SvLEN()

When determining the length of buffer needed to output the decimal point
in the current locale, use SvCUR(PL_numeric_radix_sv) rather than
SvLEN(PL_numeric_radix_sv). I presume this was a thinko in the original
commit. Using SvLEN currently seems harmless, since typically SvCUR <
SvLEN, but one could conceive a future scenario where locale info is set
using alien string buffers with SvLEN(sv) == 0.