Ignore backward jumps in computing serializer size

When defining a new topic, typically the serializer instructions that
are usually in constant memory and generated by the IDL compiler are
copied into memory managed by the Cyclone implementation.  For this it
needs to compute the size of the serializer, which the IDL compiler
doesn't provide.  It does this by effectively dry-running the
program.  (Note that it doesn't validate the program.)

All but the JSR operations move the program counter forward, but the JSR
operation can cause it to go backward instead and allows implementing
recursive types (the IDL compiler doesn't support them, but one might
decide to work around that limitation).  When dry-running the program,
following a backwards jump can cause a non-terminating loop.

The jump could potentially be to an unexplored address and so ignoring
all backwards jumps potentially means it skips part of the program.  As
this is not a validator and the program can always be arranged so that a
following a backwards jump is not relevant to computing the size
correctly, this is reasonable approximation.

Signed-off-by: Erik Boasson <eb@ilities.com>

src/core/ddsi/src/ddsi_cdrstream.c

@@ -364,6 +364,7 @@ static void dds_stream_countops1 (const uint32_t * __restrict ops, const uint32_
         break;
       }
       case DDS_OP_JSR: {
+        if (DDS_OP_JUMP (insn) > 0)
           dds_stream_countops1 (ops + DDS_OP_JUMP (insn), ops_end);
         ops++;
         break;