Fix memory corruption in rsa sign/verify programs

backport from d74c697

see #210
diff --git a/ChangeLog b/ChangeLog
index b22fdde..5d505b5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -11,6 +11,7 @@
    * Fix -Wshadow warnings (found by hnrkp) (#240)
    * Fix unused function warning when using MBEDTLS_MDx_ALT or
      MBEDTLS_SHAxxx_ALT (found by Henrik) (#239)
+   * Fix memory corruption in pkey programs (found by yankuncheng) (#210)
 
 = mbed TLS 1.3.12 released 2015-08-11