vasnprintf: Fix heap memory overrun bug.

Reported by Ben Pfaff <blp@cs.stanford.edu> in
<https://lists.gnu.org/archive/html/bug-gnulib/2018-09/msg00107.html>.

* lib/vasnprintf.c (convert_to_decimal): Allocate one more byte of
memory.
* tests/test-vasnprintf.c (test_function): Add another test.
scale10_round_decimal_long_double (long double x, int n)
{
  int e IF_LINT(= 0);
  mpn_t m;
  void *memory = decode_long_double (x, &e, &m);
  return scale10_round_decimal_decoded (e, m, memory, n);
}

scale10_round_decimal_long_double (long double x, int n)
{
  int e IF_LINT(= 0);
  mpn_t m;
  void *memory = decode_long_double (x, &e, &m);
  return scale10_round_decimal_decoded (e, m, memory, n);
}
