stagefright: fix clang build

Change-Id: I4f10e4af6f0b5fb0b90513ad516b36b444d3bd68
diff --git a/media/libstagefright/CallbackDataSource.cpp b/media/libstagefright/CallbackDataSource.cpp
index de972ce..e17fdf8 100644
--- a/media/libstagefright/CallbackDataSource.cpp
+++ b/media/libstagefright/CallbackDataSource.cpp
@@ -109,7 +109,8 @@
     }
 
     // Check if the cache satisfies the read.
-    if (mCachedOffset <= offset && offset < mCachedOffset + mCachedSize) {
+    if (mCachedOffset <= offset
+            && offset < (off64_t) (mCachedOffset + mCachedSize)) {
         if (offset + size <= mCachedOffset + mCachedSize) {
             memcpy(data, &mCache[offset - mCachedOffset], size);
             return size;