Fix memory leak in x509_crl_parse()

The memory leak call was caused by missing calls to pem_free().
diff --git a/ChangeLog b/ChangeLog
index fd7a3f5..12c541c 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -37,6 +37,9 @@
      cause buffer bound checks to be bypassed. Found by Eyal Itkin.
    * Fixed potential arithmetic overflow in mbedtls_base64_decode() that could
      cause buffer bound checks to be bypassed. Found by Eyal Itkin.
+   * Fix potential memory leak in x509_crl_parse(). The leak was caused by
+     missing calls to pem_free() in cases when a
+     POLARSSL_ERR_PEM_NO_HEADER_FOOTER_PRESENT error was encountered.
 
 = mbed TLS 1.3.18 branch 2016-10-17