Fix memory-counting error in rephist.c. Bug 18651.

changes/bug18651

@@ -0,0 +1,3 @@
+  o Minor bugfixes (assert on exit):
+    - Fix an assert-on-exit bug related to counting memory usage
+      in rephist.c. Fixes bug 18651; bugfix on 0.2.8.1-alpha.