Altering to a decimal field we get double value then store it 
that may cause data loss. 
Fix: use store_decimal() instead.



mysql-test/r/type_newdecimal.result:
  Fix for bug #24558: Increasing decimal column length causes data loss
    - test result.
mysql-test/t/type_newdecimal.test:
  Fix for bug #24558: Increasing decimal column length causes data loss
    - test case.
sql/field_conv.cc:
  Fix for bug #24558: Increasing decimal column length causes data loss
    - if target field's result type is DECIMAL_RESULT
      use store_decimal(val_decimal()) in order not to loss data.