Fix panic parsing list definition

block.go

@@ -1360,7 +1360,7 @@ gatherlines:
 			if *flags&ListTypeDefinition != 0 && i < len(data)-1 {
 				// is the next item still a part of this list?
 				next := i
-				for next < len(data) && data[next] != '\n' {
+				for next < len(data)-1 && data[next] != '\n' {
 					next++
 				}
 				for next < len(data)-1 && data[next] == '\n' {

markdown_test.go

@@ -34,6 +34,10 @@ func TestDocument(t *testing.T) {
 		// https://github.com/russross/blackfriday/issues/173
 		"   [",
 		"<p>[</p>\n",
+
+		// This should't panic.
+		"text\n\n:item: **text**\ntext\n",
+		"<dl>\n<dt>text</dt>\n</dl>\n\n<p>:item: <strong>text</strong>\ntext</p>\n",
 	}
 	doTests(t, tests)
 }