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/guile/multimap/example.i

@@ -78,7 +78,7 @@ extern int count(char *bytes, int len, char c);
 
 %typemap(argout) (char *str, int len) {
   SWIG_APPEND_VALUE(scm_from_locale_stringn($1,$2));
-  if ($1) SWIG_free($1);
+  SWIG_free($1);
 }   
 
 extern void capitalize(char *str, int len);