Don't SEGV if user returns a non string object

Make sure the object we get back on the reader function is actually a
string.  If it isn't, return an error to libxml2.

Fixes #898

ext/nokogiri/xml_io.c

@@ -21,6 +21,7 @@ int io_read_callback(void * ctx, char * buffer, int len) {
 
   if (NIL_P(string)) return 0;
   if (string == Qundef) return -1;
+  if (TYPE(string) != T_STRING) return -1;
 
   str_len = (size_t)RSTRING_LEN(string);
   safe_len = str_len > (size_t)len ? (size_t)len : str_len;

test/xml/test_reader.rb

@@ -4,6 +4,18 @@
 module Nokogiri
   module XML
     class TestReader < Nokogiri::TestCase
+      class NonStringIO
+        def read(size)
+          :invalid_object
+        end
+      end
+
+      def test_io_non_string
+        io = NonStringIO.new
+        reader = Nokogiri::XML::Reader(io)
+        assert_equal io, reader.source
+      end
+
       def test_from_io_sets_io_as_source
         io = File.open SNUGGLES_FILE
         reader = Nokogiri::XML::Reader.from_io(io)