FORMAT() can print more integer digits (than the argument has)
if rounding happens:

  FORMAT(9.9,0) -> '10'

The old code did not take this into account.

Fix:

1. One extra digit is needed in case of rounding

- If args[1] is a not-NULL constant, then reserve space for one extra integer
  digit if the requested number of decimals is less than args[0]->decimals.

- Otherwise, reserve space for one extra integer digit if
  args[0]->decimals is not 0, because rounding can potentially happen
  (depending on the exact data in arguments).

2. One extra digit is also needed if the argument has no integer digits,
   e.g. in a data type like DECIMAL(38,38).

The conditions 1 and 2 are ORed.

3. Fixing FORMAT_MAX_DECIMALS from 30 to 38. This was forgotten in 10.2.1
   (when the limit for the number of fractional digits in DECIMAL was extended).