Be more clear in changes file for 6797.

changes/bug6797

@@ -1,3 +1,5 @@
   o Minor bugfixes:
-    - Don't follow the NULL pointer. Found by "f. tp.". Fixes bug
-      6797; bugfix on 0.2.4.1-alpha.
+    - Don't follow the NULL pointer if microdescriptor generation fails.
+      (This does not appear to be triggerable, but it's best to be safe.)
+      Found by "f. tp.". Fixes bug 6797; bugfix on 0.2.4.1-alpha.
+