strfmon: Fix formatting of a second fixed-width value

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