Removing the if condition would be incorrect. The maximum char is computed at the beginning of the formatting. If, during formatting, need for a padding character is determined, the padding character must not be larger than the maximum char of the target string - which is 127 unless 'c' formatting is used.

One solution would be to determine whether the padding character is used in advance. Another solution would be to widen the string when the need for non-ASCII padding is detected.

I have no intention of fixing this issue myself; I don't mind if non-ASCII padding characters are not supported in 3.3.