csn1: fix M_CHOICE: restirct maximum length of the choice list

The current implementation is not capable of handling more than
256 (UCHAR_MAX) selectors in the choice list. Let's document
this and add a guard check to the M_CHOICE handler.

Change-Id: I40c3c5b9be892804c6cd71cbb907af469ce5d769

src/csn1.c

@@ -495,6 +495,8 @@ csnStreamDecoder(csnStream_t* ar, const CSN_DESCR* pDescr, struct bitvec *vector
         /* Make sure that the list of choice items is not empty */
         if (!count)
           return ProcessError(readIndex, "csnStreamDecoder", CSN_ERROR_IN_SCRIPT, pDescr);
+        else if (count > 255) /* We can handle up to 256 (UCHAR_MAX) selectors */
+          return ProcessError(readIndex, "csnStreamDecoder", CSN_ERROR_IN_SCRIPT, pDescr);
 
         while (count > 0)
         {

src/csn1.h

@@ -490,7 +490,8 @@ gint16 csnStreamEncoder(csnStream_t* ar, const CSN_DESCR* pDescr, struct bitvec
  * is the part of the message.  In the CSN_CHOICE case, this rule does not
  * apply. There is free but predefined mapping of the element of the union and
  * the value which addresses this element.
- * The value of the address is called a selector.
+ * The value of the address is called a selector. Up to 256 (UCHAR_MAX) unique
+ * selectors can be handled, longer choice list would cause CSN_ERROR_IN_SCRIPT.
  * After unpacking, this value is then converted to the sequential number of the
  * element in the union and stored in the UnionType variable.
  *      Par1: C structure name