genhtml: Fix warning with small genhtml_line_field_width

On systems with Perl versions 5.21 and above, genhtml prints a warning
similar to the following during processing:

  genhtml: Negative repeat count does nothing at bin/genhtml line 3854,
           <SOURCE_HANDLE> line 4.

This is due to size calculations resulting in a negative number of
padding characters when genhtml_line_field_width is lower than the size
of the strings to pad (9). Fix this by disabling padding in these cases.

Reported-by: xaizek@openmailbox.org
Signed-off-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>

Author: oberpar

bin/genhtml

@@ -3851,8 +3851,8 @@ sub fmt_centered($$)
 {
 	my ($width, $text) = @_;
 	my $w0 = length($text);
-	my $w1 = int(($width - $w0) / 2);
-	my $w2 = $width - $w0 - $w1;
+	my $w1 = $width > $w0 ? int(($width - $w0) / 2) : 0;
+	my $w2 = $width > $w0 ? $width - $w0 - $w1 : 0;
 
 	return (" "x$w1).$text.(" "x$w2);
 }