Fix possible unlock before lock in RSA

Backport of 1385a28 and 4d04cdc

see #257
diff --git a/ChangeLog b/ChangeLog
index b61c2c5..43dd6b2 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -5,6 +5,9 @@
 Bugfix
    * Setting SSL_MIN_DHM_BYTES in config.h had no effect (overriden in ssl.h)
      (found by Fabio Solari) (#256)
+   * Fix bug in mbedtls_rsa_public() and mbedtls_rsa_private() that could
+     result trying to unlock an unlocked mutex on invalid input (found by
+     Fredrik Axelsson) (#257)
 
 = mbed TLS 1.3.12 released 2015-08-11