Auto merge of #24269 - benashford:formatting-fix, r=alexcrichton

Auto merge of #24269 - benashford:formatting-fix, r=alexcrichton

This fixes the bug described in issue #23150.  This affected formatting any floating point number into a string in a formatting pattern that: a) required rounding up, and b) required an extra digit on the front.

So `format!("{:.0}", 9.9)` would fail, but `format!("{:.0}", 8.9)` would succeed.  This was due to a negative integer being cast to a `usize` resulting in an 'arithmetic operation overflowed' panic.

The fix was to change the order of operations so that the number is zero before casting.