IFNULL is used with type combination {decimal,int}, the resulting
decimal loses on digit after the decimal point in the calculation of
::max_length. This leads to a loss of a decimal digit after point in
the corner case of GROUP BY which needs a temporary file, e.g. this
kind of query:

   SELECT IFNULL(e2,i1) in, SUM(d1) FROM t GROUP BY e2,i1 ORDER BY in;

This bug will be more visible when we get window functions which rely on
temporary files in non-trivial cases.

FIX
------------------------------

Add space for decimal point to the calculation in
Item_func_ifnull::resolve_type.

Repro test added.

ANALYSIS (refer to added test case):
------------------------------------

The calculation of char_length in Item_func_ifnull::resolve_type is
used as an argument to fix_char_length. The calculation forgets to add
space for decimal point if present.

The calculation for NULLIF looks like this:

Item_func_ifnull::resolve_type:
   :
   char_length= max(len0, len1) + decimals + (unsigned_flag ? 0 : 1);

where "decimals" is 2 in our case.

In the similar computation of IF, this is calculated with another (correct)
code path:

cf Item_func_if::resolve_type ->
   fix_num_type_shared_for_case->
   Item_func::count_decimal_length ->
   my_decimal_precision_to_length_no_truncation

which has this line:
   uint32 retval= (uint32)(precision + (scale > 0 ? 1 : 0) +
                  (unsigned_flag || !precision ? 0 : 1));

the "scale" here being the number of digits after point, making the
calculation add 1 in our case, since "scale" is 2.

Change-Id: If54750e9e3b456d063591a359514d69f2bc0c510