rb_sandbox_init() should check whether value of argument is String. B…

…efore changing, segfault occurred with Sandbox.new(Fixnum).

Test:
{{{
$ macruby -e 'Sandbox.new(1)'
}}}

git-svn-id: http://svn.macosforge.org/repository/ruby/MacRuby/trunk@5139 23306eb0-4c56-4727-a40e-e92c0eb68959

sandbox.c

@@ -31,6 +31,7 @@ rb_sandbox_init(VALUE obj, SEL sel, VALUE profile)
 {
     rb_sandbox_t *box;
 
+    StringValue(profile);
     Data_Get_Struct(obj, rb_sandbox_t, box);
     GC_WB(&box->profile, ruby_strdup(RSTRING_PTR(profile)));
     box->flags = 0;