Reserve the space for the leading 0 (before the decimal point) in DECIMAL(a,a) ZEROFILL.


mysql-test/r/type_decimal.result:
  show that we allocate a large enough buffer for output of DECIMAL(a,a) [ZEROFILL].
mysql-test/t/type_decimal.test:
  show that we allocate a large enough buffer for output of DECIMAL(a,a) [ZEROFILL].
  without patch for bug#31227, valgrind will complain here; so will a
  debug build.
sql/my_decimal.cc:
  Reserve the space for the leading 0 (before the decimal point) in DECIMAL(a,a) ZEROFILL.
  Doxygenise preamble of my_decimal2string().