message: Fix a crash with $param() filtering on fake markers
diff: @@ -1740,6 +1740,9 @@ searched for as a case-insensitive substring.
  return
  [paramName, paramValue](ICaptureContext *ctx, const rdcstr &, const rdcstr &, uint32_t,
  const SDChunk *chunk, const DrawcallDescription *, const rdcstr &) {
  + if(!chunk)
  + return false;
  +
  const SDObject *o = FindChildRecursively(chunk, paramName);
  if(!o)
  