Add a changelog for the STACK fixes

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+  o Minor bugfixes (correctness):
+    - Fix several places in our codebase where a C compiler would be likely
+      to eliminate a check, based on assuming that undefined behavior had not
+      happened elsewhere in the code.  These cases are usually a sign of
+      redundant checking, or dubious arithmetic. Found by Georg Koppen using
+      the "STACK" tool from Wang, Zeldovich, Kaashoek, and
+      Solar-Lezama. Fixes bug 24423; bugfix on various Tor versions.