Don't say "downloading 1 descriptor, 4 at a time"

Fixes bug 19648, bugfix on 0.1.1.8-alpha.

changes/bug19648

@@ -0,0 +1,5 @@
+  o Minor bugfixes (logging):
+    - When logging the number of descriptors we intend to download per
+      directory request, do not log a number higher than then the
+      number of descriptors we're fetching in total. Fixes bug 19648;
+      bugfix on 0.1.1.8-alpha.

src/or/routerlist.c

@@ -5054,8 +5054,9 @@ launch_descriptor_downloads(int purpose,
   if (n_per_request > max_dl_per_req)
     n_per_request = max_dl_per_req;
 
-  if (n_per_request < MIN_DL_PER_REQUEST)
-    n_per_request = MIN_DL_PER_REQUEST;
+  if (n_per_request < MIN_DL_PER_REQUEST) {
+    n_per_request = MIN(MIN_DL_PER_REQUEST, n_downloadable);
+  }
 
   if (n_downloadable > n_per_request)
     req_plural = rtr_plural = "s";