Better handle null pointer checks in GPRE generated code

[Noremos]

A static analyzer (such as Clang, for example) prints many warnings in GPRE-generated code:

for (int retries = 0; retries < 2; ++retries)
{
  if (!req_handle1 && DB)
		  req_handle1 = DB->compileRequest(fbStatus, sizeof(fb_618), fb_618);
  if (req_handle1)
		  req_handle1->start(fbStatus, fbTrans, 0);
  if (fbStatus->getErrors()[1] == isc_bad_req_handle) { req_handle1->release(); req_handle1 = NULL; }  <- Called C++ object pointer is null
  else break;
}
if (!(fbStatus->getState() & Firebird::IStatus::STATE_ERRORS)) {
  while (1)
  {
  req_handle1->receive (fbStatus, 0, 0, 1034, CAST_MSG(&fb_619)); <- Called C++ object pointer is null
  ....

There are a lot of these warnings, so I suggest 2 changes in the code generation to fix them

for (int retries = 0; retries < 2; ++retries)
{
   if (!req_handle1 && DB)
	  req_handle1 = DB->compileRequest(fbStatus, sizeof(fb_25), fb_25);
  if (req_handle1)
  {
	  req_handle1->start(fbStatus, fbTrans, 0);
	  if (fbStatus->getErrors()[1] == isc_bad_req_handle) { req_handle1->release(); req_handle1 = NULL; }
	  else break;
  }
}
if (req_handle1 && !(fbStatus->getState() & Firebird::IStatus::STATE_ERRORS)) {
  while (1)
  {
  req_handle1->receive (fbStatus, 0, 0, 1034, CAST_MSG(&fb_619)); <- Called C++ object pointer is null
  ....