Remove unneeded NULL checks

It's perfectly safe to delete NULL pointers, so simplify things by
removing these checks.

common/rfb/CSecurityStack.cxx

 
 CSecurityStack::~CSecurityStack()
 {
-  if (state0)
-    delete state0;
-  if (state1)
-    delete state1;
+  delete state0;
+  delete state1;
 }
 
 bool CSecurityStack::processMsg()

common/rfb/CSecurityVeNCrypt.cxx

 
 CSecurityVeNCrypt::~CSecurityVeNCrypt()
 {
-  if (availableTypes)
-	delete[] availableTypes;
+  delete[] availableTypes;
 }
 
 bool CSecurityVeNCrypt::processMsg()

common/rfb/SSecurityStack.cxx

 
 SSecurityStack::~SSecurityStack()
 {
-  if (state0)
-    delete state0;
-  if (state1)
-    delete state1;
+  delete state0;
+  delete state1;
 }
 
 bool SSecurityStack::processMsg()

common/rfb/SSecurityVeNCrypt.cxx

 SSecurityVeNCrypt::~SSecurityVeNCrypt()
 {
   delete ssecurity;
-
-  if (subTypes) {
-    delete [] subTypes;
-    subTypes = NULL;
-  }
+  delete [] subTypes;
 }
 
 bool SSecurityVeNCrypt::processMsg()