Set next sequence of subject_alt_names to NULL

Set the next sequence of the subject_alt_name to NULL when deleting
sequence on failure in `get_subject_alt_name()`.
Found by Philippe Antoine. Credit to OSS-Fuzz.
diff --git a/ChangeLog b/ChangeLog
index d333692..6ac3e78 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -46,6 +46,9 @@
      for the parameter.
    * Add a check for MBEDTLS_X509_CRL_PARSE_C in ssl_server2, guarding the crl
      sni entry parameter. Reported by inestlerode in #560.
+   * Set the next sequence of the subject_alt_name to NULL when deleting
+     sequence on failure. Found and fix suggested by Philippe Antoine.
+     Credit to OSS-Fuzz.
 
 API Changes
    * Extend the MBEDTLS_SSL_EXPORT_KEYS to export the handshake randbytes,