THRIFT-572. rb: fix RSTRING for 1.9 compatibility

Use a macro compatible with Ruby 1.9.



git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@810303 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/lib/rb/ext/memory_buffer.c b/lib/rb/ext/memory_buffer.c
index 3a9c101..74efa2c 100644
--- a/lib/rb/ext/memory_buffer.c
+++ b/lib/rb/ext/memory_buffer.c
@@ -54,7 +54,7 @@
     index = 0;
   }
 
-  if (RSTRING(data)->len < length) {
+  if (RSTRING_LEN(data) < length) {
     rb_raise(rb_eEOFError, "Not enough bytes remain in memory buffer");
   }