ruby.c: do not modify shared string

* ruby.c (ruby_init_loadpath_safe): get rid of modify shared
  string directly.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48742 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ruby.c

@@ -459,7 +459,9 @@ ruby_init_loadpath_safe(int safe_level)
 	    p = p2;
 	}
 #endif
+#if !VARIABLE_LIBPATH
 	*p = 0;
+#endif
     }
 #if !VARIABLE_LIBPATH
     else {