(perl #126203) avoid potential leaks on quadmath_snprintf() failure

(perl #126203) avoid potential leaks on quadmath_snprintf() failure

In the unlikely case quadmath_snprintf() fails both sv_vcatpvfn_flags()
and my_snprintf() could leak the temp format string returned by
quadmath_format_single() if quadmath_format_single() had to rebuild
the format.

Getting quadmath_snprintf() to fail in this context seems impractical,
but future changes may make it happen, so clean up after ourselves.