rdmult data type change to fix high bit-depth

Fixes an intermittent assert failure for highbitdepth.

Change-Id: If8cad0209a94f1184b69c7b3f1d587934f857d9b

vp9/encoder/vp9_rd.c

diff --git a/vp9/encoder/vp9_rd.c b/vp9/encoder/vp9_rd.c
index a32776ac..efb8686 100644
--- a/vp9/encoder/vp9_rd.c
+++ b/vp9/encoder/vp9_rd.c
@@ -136,7 +136,7 @@
 };
 
 int vp9_compute_rd_mult(const VP9_COMP *cpi, int qindex) {
-  const int q = vp9_dc_quant(qindex, 0, cpi->common.bit_depth);
+  const int64_t q = vp9_dc_quant(qindex, 0, cpi->common.bit_depth);
 #if CONFIG_VP9_HIGHBITDEPTH
   int rdmult = 0;
   switch (cpi->common.bit_depth) {