fix: Stop panicking when the start directive skips directly to the end directive.

goldens/skip_lines.in

@@ -1,3 +1,7 @@
+Skip lines with an empty block:
+keep-sorted-test start skip_lines=1
+keep-sorted-test end
+
 Skip two lines:
 // keep-sorted-test start skip_lines=2
 foo

goldens/skip_lines.out

@@ -1,3 +1,7 @@
+Skip lines with an empty block:
+keep-sorted-test start skip_lines=1
+keep-sorted-test end
+
 Skip two lines:
 // keep-sorted-test start skip_lines=2
 foo

keepsorted/block.go

@@ -114,7 +114,7 @@ func (f *Fixer) newBlocks(filename string, lines []string, offset int, include f
 			}
 
 			start.index += opts.SkipLines
-			if start.index > endIndex {
+			if start.index >= endIndex {
 				continue
 			}