Adding decimal "digits" in multiplication resulted in signed overflow and
producing wrong results.

  Fixed by using large enough buffers and intermediary result types :
dec2 (currently longlong) to hold result of adding decimal "digits" 
(currently int32). 


mysql-test/r/select.result:
  Bug #20569 Garbage in DECIMAL results from some mathematical functions
    * test suite for the bug
mysql-test/t/select.test:
  Bug #20569 Garbage in DECIMAL results from some mathematical functions
    * test suite for the bug
strings/decimal.c:
  Bug #20569 Garbage in DECIMAL results from some mathematical functions
    * fixed the overflow in adding decimal "digits"