Converting the number zero to binary and back yielded the number zero,
but with no digits, i.e. zero precision.
This made the multiply algorithm go haywire in various ways.


include/decimal.h:
  Document struct st_decimal_t
mysql-test/r/type_newdecimal.result:
  New test case (valgrind warnings)
mysql-test/t/type_newdecimal.test:
  New test case (valgrind warnings)
sql/my_decimal.h:
  Remove the HAVE_purify enabled/disabled code.
strings/decimal.c:
  Make a proper zero, with non-zero precision.