Remove redundant NULL checks before free()/delete (#2184)

Remove redundant NULL checks before free()/delete

The ISO C and C++ standards guarantee that it's safe to call these
on a NULL pointer, so it's not necessary for the calling code to
also check.

Fixes https://sourceforge.net/p/swig/feature-requests/70/

Lib/typemaps/cpointer.swg

@@ -55,7 +55,7 @@ typedef struct {
     return %new_instance(TYPE);
   }
   ~NAME() {
-    if ($self) %delete($self);
+    %delete($self);
   }
 }
 
@@ -114,7 +114,7 @@ typedef struct {
   }
 
   static void delete_##NAME(TYPE *obj) { 
-    if (obj) %delete(obj);
+    %delete(obj);
   }
 
   static void NAME ##_assign(TYPE *obj, TYPE value) {