Is the proposed solution really the best approach? It seems complicated to me and relies on obscure parts of the language. Maybe the problem (compiler optimizes away function call because the result is no longer needed) could be solved like this:

  memset(key, 0, sizeof(key)); 
  if (key[0])  // we are using key, so you can't skip memset()
    dropDead();

Unless the compilers "understand" memset and still optimize away the last two lines? I would hope not... Does anyone know how aggressive the C optimizers are these days?