When calculating the result length of an integer DIV function 
the number of decimals was used without checking the result type
first. Thus an uninitialized number of decimals was used for some 
types. This caused an excessive amount of memory to be allocated 
for the field's buffer and crashed the server.

 
Fixed by using the number of decimals only for data types that 
can have decimals and thus have valid decimals number.


mysql-test/r/func_math.result:
  Bug #30587: test case
mysql-test/t/func_math.test:
  Bug #30587: test case
sql/item_func.cc:
  Bug #30587: Don't use decimals on a type that doesn't have them.