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This is a backport of the patch for MDEV-9653 (fixed earlier in 10.1.13).

The code in Item_func_case::fix_length_and_dec() did not
calculate max_length and decimals properly.

In case of any numeric result (DECIMAL, REAL, INT) a generic method
Item_func_case::agg_num_lengths() was called, which could erroneously result
into a DECIMAL item with max_length==0 and decimals==0, so the constructor of
Field_new_decimals tried to create a field of DECIMAL(0,0) type,
which caused a crash.

Unlike Item_func_case, the code responsible for merging attributes in
Item_func_coalesce::fix_length_and_dec() works fine: it has specific execution
branches for all distinct numeric types and correctly creates a DECIMAL(1,0)
column instead of DECIMAL(0,0) for the same set of arguments.

The fix does the following:
- Moves the attribute merging code from Item_func_coalesce::fix_length_and_dec()
  to a new method Item_func_hybrid_result_type::f...