When a zero length is provided to the my_decimal_length_to_precision
function along with unsigned_flag set to false it returns a negative value.
For queries that employs temporary tables may cause failed assertion or
excessive memory consumption while temporary table creation.

Now the my_decimal_length_to_precision and the my_decimal_precision_to_length
functions take unsigned_flag into account only if the length/precision
argument is non-zero.


mysql-test/t/type_decimal.test:
  Added a test case for the bug#36023: Incorrect handling of zero length caused
   an assertion to fail.
mysql-test/r/type_decimal.result:
  Added a test case for the bug#36023: Incorrect handling of zero length caused
   an assertion to fail.
sql/my_decimal.h:
  Bug#36023: Incorrect handling of zero length caused an assertion to fail.
  Now the my_decimal_length_to_precision and the my_decimal_precision_to_length
  functions take unsigned_flag into account only if the length/precision
  argument is non-zero.