Bug#12608543: CRASHES WITH DECIMALS AND STATEMENT NEEDS TO BE REPREPARED ERRORS

Backporting these two fixes to 5.1 
Added unittest to test my_decimal construtor and assignment operators

sql/my_decimal.h:
  Added constructor and assignment operators for my_decimal
unittest/my_decimal/my_decimal-t.cc:
  Added test to check constructor and assignment operators for my_decimal