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/guile/std_common.i

@@ -19,7 +19,7 @@ std::string SWIG_scm2string(SCM x) {
     char* temp;
     temp = SWIG_scm2str(x);
     std::string s(temp);
-    if (temp) SWIG_free(temp);
+    SWIG_free(temp);
     return s;
 }
 %}