Chaos: handle failed allocation in get_media_encryption_type

If the ast_strndup() call fails to allocate a copy of the
transport string for parsing, fail gracefully.

ASTERISK-25323
Reported by: Scott Griepentrog

Change-Id: Ia4b905ce6d03da53fea526224455c1044b1a5a28

res/res_pjsip_sdp_rtp.c

@@ -602,6 +602,9 @@ static enum ast_sip_session_media_encryption get_media_encryption_type(pj_str_t
 
 	*optimistic = 0;
 
+	if (!transport_str) {
+		return AST_SIP_MEDIA_TRANSPORT_INVALID;
+	}
 	if (strstr(transport_str, "UDP/TLS")) {
 		return AST_SIP_MEDIA_ENCRYPT_DTLS;
 	} else if (strstr(transport_str, "SAVP")) {