There were some misunderstandings about parameters pertaining to buffer-size.

Patches fixes the reported off by one and
clarifies the documentation.

mysql-test/r/type_newdecimal.result:
  add test
mysql-test/t/type_newdecimal.test:
  add test
sql/field.cc:
  adjust buffer size by one to account for terminator.
sql/my_decimal.cc:
  adjust buffer size by one to account for terminator.
  clarify needs in comments.
sql/my_decimal.h:
  clarify buffer-size needs to prevent future off-by-one bugs.
strings/decimal.c:
  clarify buffer-size needs and parameters to prevent future off-by-one bugs