strfmon: Fix formatting of a second fixed-width value

There is a bug when formatting two consecutive values using fixed-widths
and the values need padding. This was because the value of pad_size
was zeroed only every other time.

Format Before After
[%8n] [%8n] [ $123.45] [ $123.45] [ $123.45] [ $123.45]

Reviewed by: kib
PR: 267282
Github PR: #619
MFC after: 1 week