Eliminate unnecessary NULL checks before tjFree()

+ document that tjFree() accepts NULL pointers without complaint.
Effectively, it has had that behavior all along, but the API does not
guarantee that tjFree() will be implemented with free() behind the
scenes, so it's best to formalize the behavior.

doc/html/group___turbo_j_p_e_g.html

@@ -2078,7 +2078,7 @@ If you choose option 1, <code>*jpegSize</code> should be set to the size of your
 <p>You should always use this function to free JPEG destination buffer(s) that were automatically (re)allocated by the compression and transform functions or that were manually allocated using <a class="el" href="group___turbo_j_p_e_g.html#gaec627dd4c5f30b7a775a7aea3bec5d83" title="Allocate an image buffer for use with TurboJPEG.">tjAlloc()</a>.</p>
 <dl class="params"><dt>Parameters</dt><dd>
   <table class="params">
-    <tr><td class="paramname">buffer</td><td>address of the buffer to free</td></tr>
+    <tr><td class="paramname">buffer</td><td>address of the buffer to free. If the address is NULL, then this function has no effect.</td></tr>
   </table>
   </dd>
 </dl>