Revise fix for bug 32178 (spaces at end of log msg).

The loop in the earlier patch would invoke undefined behavior in two
ways: First, it would check whether it was looking at a space before
it checked whether the pointer was in-range.  Second, it would let a
pointer reach a position _before_ the start of a string, which is
not allowed.

I've removed the assertion about empty messages: empty messages can
be their own warning IMO.

I've also added tests for this formatting code, to make sure it
actually works.

changes/ticket32178

@@ -1,3 +1,3 @@
   o Minor bugfixes (logging):
     - Remove trailing whitespaces from control event log messages. Fixes bug
-      32178.
+      32178; bugfix on 0.1.1.1-alpha. Based on a patch by Amadeusz Pawlik.