fix warning from scalar --> int32 conversion

BUG=

Review URL: https://codereview.appspot.com/7065050

git-svn-id: http://skia.googlecode.com/svn/trunk@7061 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/xml/SkBML_XMLParser.cpp b/src/xml/SkBML_XMLParser.cpp
index dbdfe4b..bf49782 100644
--- a/src/xml/SkBML_XMLParser.cpp
+++ b/src/xml/SkBML_XMLParser.cpp
@@ -15,7 +15,7 @@
 static uint8_t rbyte(SkStream& s)
 {
     uint8_t b;
-    size_t size = s.read(&b, 1);
+    SkDEBUGCODE(size_t size = ) s.read(&b, 1);
     SkASSERT(size == 1);
     return b;
 }