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/

Examples/test-suite/namespace_typemap.i

@@ -15,7 +15,7 @@ namespace test {
 	strcpy(data,s);
       }
      ~string_class() {
-        if (data) delete [] data;
+        delete [] data;
       }
       char *c_str() {
         return data;