Don't send missing X-Desc-Gen-Reason on startup

Since we start with desc_clean_since = 0, we should have been
starting with non-null desc_dirty_reason.

Fixes bug 22884; bugfix on 0.2.3.4-alpha when X-Desc-Gen-Reason was
added.

changes/bug22885

@@ -0,0 +1,5 @@
+  o Minor bugfixes (relay):
+    - When uploading our descriptor for the first time after startup,
+      report the reason for uploading as "Tor just started" rather than
+      leaving it blank. Fixes bug 22885; bugfix on 0.2.3.4-alpha.
+

src/or/router.c

@@ -1856,7 +1856,7 @@ static const char *desc_gen_reason = NULL;
  * now. */
 static time_t desc_clean_since = 0;
 /** Why did we mark the descriptor dirty? */
-static const char *desc_dirty_reason = NULL;
+static const char *desc_dirty_reason = "Tor just started";
 /** Boolean: do we need to regenerate the above? */
 static int desc_needs_upload = 0;