stun: check identifier before using it

By default, 'candidate_identifier == NULL' only happens for 'compatibility
== NICE_COMPATIBILITY_GOOGLE'. However, keepalive=true will also trigger
the same code path so candidate_identifier must be checked to avoid a
segfault.
1 job for keepalive in 1 minute and 16 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #31613
build-and-test

00:01:16