library/MNCC_EncDec.cc: fix encoding of optional 'sdp' field

Decoding was implemented in [1], but encoding was not.

Change-Id: I82f8afd3e64d83d165a7070a1b82a6276ef0b020
Fixes: [1] Ic9568c8927507e161aadfad1a4d20aa896d8ae30
diff --git a/library/MNCC_EncDec.cc b/library/MNCC_EncDec.cc
index 7d09d0a..f896cd3 100644
--- a/library/MNCC_EncDec.cc
+++ b/library/MNCC_EncDec.cc
@@ -184,6 +184,10 @@
 		strncpy(mncc.imsi, in_sig.imsi(), sizeof(mncc.imsi));
 		mncc.lchan_type = in_sig.lchan__type();
 		mncc.lchan_mode = in_sig.lchan__mode();
+		if (in_sig.sdp().is_value()) {
+			const CHARSTRING &sdp = in_sig.sdp();
+			strncpy(&mncc.sdp[0], sdp, sizeof(mncc.sdp));
+		}
 		ret_val = OCTETSTRING(sizeof(mncc), (uint8_t *)&mncc);
 		}
 		break;