Add check for validity of date in x509_get_time()

ChangeLog

diff --git a/ChangeLog b/ChangeLog
index 60383a9..a63957c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -29,6 +29,8 @@
      a contribution from Tobias Tangemann. #541
    * Fixed cert_app sample program for debug output and for use when no root
      certificates are provided.
+   * Fix check for validity of date when parsing in mbedtls_x509_get_time().
+     Found by subramanyam-c.
 
 Changes
    * Extended test coverage of special cases, and added new timing test suite.