Fix overflow in RD error calculation code.

Change-Id: I61ef1f198c876f9f79787ea7d7385a862cfbae19

vp9/encoder/vp9_rdopt.c

diff --git a/vp9/encoder/vp9_rdopt.c b/vp9/encoder/vp9_rdopt.c
index f971d91..990be65 100644
--- a/vp9/encoder/vp9_rdopt.c
+++ b/vp9/encoder/vp9_rdopt.c
@@ -603,7 +603,7 @@
 
   for (i = 0; i < block_size; i++) {
     int this_diff = coeff[i] - dqcoeff[i];
-    error += this_diff * this_diff;
+    error += (unsigned)this_diff * this_diff;
   }
   error >>= shift;