Fix bug discovered by John W. Shipman -- when the width of a format

Fix bug discovered by John W. Shipman -- when the width of a format
specifier came from an int expression instead of a constant in the
format, a negative width was truncated to zero instead of taken to
mean the same as that negative constant plugged into the format.  E.g.
"(%*s)" % (-5, "foo") yielded "(foo)" while "(%-5s)" yields "(foo  )".
Now both yield the latter -- like sprintf() in C.