convolve_test: Fix high bit depth IOC runtime errors.

Add a cast.

BUG=webm:1225

Change-Id: I34ea18ee816569485c1f1046a81fd2a0ce527ac8
diff --git a/test/convolve_test.cc b/test/convolve_test.cc
index 9c71f23..73b0edb 100644
--- a/test/convolve_test.cc
+++ b/test/convolve_test.cc
@@ -231,7 +231,8 @@
    * and filter_max_width = 16
    */
   uint16_t intermediate_buffer[71 * kMaxDimension];
-  const int intermediate_next_stride = 1 - intermediate_height * output_width;
+  const int intermediate_next_stride =
+      1 - static_cast<int>(intermediate_height * output_width);
 
   // Horizontal pass (src -> transposed intermediate).
   {