Fix index out of range in headline priority parsing

fuzzed - counting is hard...

org/headline.go

@@ -56,7 +56,7 @@ func (d *Document) parseHeadline(i int, parentStop stopFn) (int, Node) {
 		}
 	}
 
-	if len(text) >= 3 && text[0:2] == "[#" && strings.Contains("ABC", text[2:3]) && text[3] == ']' {
+	if len(text) >= 4 && text[0:2] == "[#" && strings.Contains("ABC", text[2:3]) && text[3] == ']' {
 		headline.Priority = text[2:3]
 		text = strings.TrimSpace(text[4:])
 	}

org/testdata/misc.html

@@ -60,7 +60,17 @@
 </li>
 </ul>
 </li>
-<li><a href="#headline-27">Footnotes</a>
+<li><a href="#headline-27">misc fuzz / regression / edge case</a>
+<ul>
+<li><a href="#headline-28">index out of range in headline priority parsing</a>
+<ul>
+<li><a href="#headline-29">[#B</a>
+</li>
+</ul>
+</li>
+</ul>
+</li>
+<li><a href="#headline-30">Footnotes</a>
 </li>
 </ul>
 </nav>
@@ -480,7 +490,16 @@ When inserting an image link like <img src="/home/amos/Pictures/Screenshots/img-
 </pre>
 </div>
 </div>
-<h1 id="headline-27">
+<h2 id="headline-27">
+misc fuzz / regression / edge case
+</h2>
+<h3 id="headline-28">
+index out of range in headline priority parsing
+</h3>
+<h4 id="headline-29">
+[#B
+</h4>
+<h1 id="headline-30">
 Footnotes
 </h1>
 <div class="footnotes">

org/testdata/misc.org

@@ -133,6 +133,9 @@ When inserting an image link like [[/home/amos/Pictures/Screenshots/img-2017-09-
       (shrink-window (- (window-height) 12))
       (ansi-term))
     #+END_SRC
+** misc fuzz / regression / edge case
+*** index out of range in headline priority parsing
+**** [#B
 * Footnotes
 
 [fn:1] a footnote /with/ *markup*

org/testdata/misc.pretty_org

@@ -133,6 +133,9 @@ When inserting an image link like [[/home/amos/Pictures/Screenshots/img-2017-09-
   (shrink-window (- (window-height) 12))
   (ansi-term))
 #+END_SRC
+** misc fuzz / regression / edge case
+*** index out of range in headline priority parsing
+**** [#B
 * Footnotes
 
 [fn:1] a footnote /with/ *markup*