output UTF-8 decoded id-on-xmppAddr SAN's
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gnutls_x509_crt_get_subject_alt_name() makes a promise  "If an otherName OID is known, the data will be decoded. ... RFC 3920 id-on-xmppAddr SAN is recognized." which it didn't hold.
Before this patch the output was still in DER format, e.g. for a id-on-xmppAddr which is always UTF-8 (0x0c):
0x0c <len> <xmppAddr>
This patch fixes the issue and now it returns the decoded string.