MNCC: Add input validation

There appears to have been no input validation whatsoever on MNCC
messages.  Hence it was very easy for an external MNCC handler to
crash OsmoMSC, such as in OS#2853

Change-Id: Idaf3b8e409c84564b1eb26d01a19c605f89b14f4
Closes: OS#2853
diff --git a/include/osmocom/msc/mncc.h b/include/osmocom/msc/mncc.h
index 49f0c8b..d2f0541 100644
--- a/include/osmocom/msc/mncc.h
+++ b/include/osmocom/msc/mncc.h
@@ -215,5 +215,6 @@
 		|| msg_type == GSM_TCH_FRAME_AMR \
 		|| msg_type == GSM_BAD_FRAME)
 
+int mncc_prim_check(const struct gsm_mncc *mncc_prim, unsigned int len);
 
 #endif