Drop initial null check from SafeFree

Both free and delete treat nullptr the same way: as a no-op [1][2].

Get rid of checking if the pointer passed is a null one before acting on
it.

See stackoverflow.com/q/154136/do-while-and-if-else-statements-in in
case you are wondering why is it done this way.

[1] http://www.cplusplus.com/reference/cstdlib/free/
[2] http://www.cplusplus.com/reference/new/operator%20delete/

src/common.h

@@ -1,9 +1,9 @@
 /*****************************************************************************
  * Copyright (c) 2014 Ted John
  * OpenRCT2, an open source clone of Roller Coaster Tycoon 2.
- * 
+ *
  * This file is part of OpenRCT2.
- * 
+ *
  * OpenRCT2 is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
  * the Free Software Foundation, either version 3 of the License, or
@@ -17,21 +17,21 @@
  * You should have received a copy of the GNU General Public License
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  *****************************************************************************/
- 
+
 #ifndef _COMMON_H_
 #define _COMMON_H_
 
 #include "diagnostic.h"
 #include "rct2.h"
 
-#define SafeFree(x) if ((x) != NULL) { free(x); (x) = NULL; }
+#define SafeFree(x) do { free(x); (x) = NULL; } while (0)
 
-#define SafeDelete(x) if ((x) != nullptr) { delete (x); (x) = nullptr; }
-#define SafeDeleteArray(x) if ((x) != nullptr) { delete[] (x); (x) = nullptr; }
+#define SafeDelete(x) do { delete (x); (x) = nullptr; } while (0)
+#define SafeDeleteArray(x) do { delete[] (x); (x) = nullptr; } while (0)
 
 #ifndef interface
 	#define interface struct
 #endif
 #define abstract = 0
 
-#endif
+#endif