csn1: Update M_NULL CSN_DESCR to match wireshark

Port from iwireshark.git commit cc6d4341e65ef2e8d8488fe0ac0f236ece0dd844.
It looks like it makes no difference to us now, but other EGPRS messages
may use it in the future.

Ported-by: Pau Espin Pedrol <pespin@sysmocom.de>
Change-Id: I34039370c292e62790a38abb59f55c69fffa88e8
diff --git a/src/csn1.h b/src/csn1.h
index 1015e69..39e6625 100644
--- a/src/csn1.h
+++ b/src/csn1.h
@@ -592,8 +592,8 @@
 #define M_PADDING_BITS(_STRUCT)\
         {CSN_PADDING_BITS, 0, {0}, 0, TRUE, "Padding", {(StreamSerializeFcn_t)0}}
 
-#define M_NULL(_STRUCT, _MEMBER)\
-        {CSN_NULL, 0, {0}, offsetof(_STRUCT, _MEMBER), FALSE, #_MEMBER, {(StreamSerializeFcn_t)0}}
+#define M_NULL(_STRUCT, _MEMBER, _SKIP_BITS)\
+        {CSN_NULL, _SKIP_BITS, {0}, offsetof(_STRUCT, _MEMBER), FALSE, #_MEMBER, {(StreamSerializeFcn_t)0}}
 
 #define M_THIS_EXIST(_STRUCT)\
         {CSN_EXIST, 0, {0}, offsetof(_STRUCT, Exist), FALSE, "Exist", {(StreamSerializeFcn_t)0}}