Fix: copy my_decimal by value, to avoid dangling pointers.


mysql-test/r/func_math.result:
  New test case.
mysql-test/t/func_math.test:
  New test case.
sql/item_cmpfunc.cc:
  No need to call fix_buffer_pointer() anymore.
sql/item_func.cc:
  Copy my_decimal by value, to avoid dangling pointers.
sql/my_decimal.h:
  Implement proper copy constructor and assignment operator for my_decimal.
sql/sql_analyse.cc:
  No need to call fix_buffer_pointer() anymore.
strings/decimal.c:
  Remove #line directive: it messes up TAGS and it confuses gdb when debugging.