cdef_alloc_data: fix sizeof in allocation

sb_index was oversized on platforms with sizeof(int*) > sizeof(int).

since:
722ad8f527 Refactor cdef search to facilitate multithreading

Bug: aomedia:3454
Change-Id: I8aae1ea0b297f17ecf933c6b21e9dea10f450b4c
(cherry picked from commit 129c0811db78e8b3264ad8d66edbb590edbc4739)
diff --git a/av1/encoder/pickcdef.c b/av1/encoder/pickcdef.c
index 22a4557..ba0d05d 100644
--- a/av1/encoder/pickcdef.c
+++ b/av1/encoder/pickcdef.c
@@ -638,7 +638,7 @@
   const int nvfb = cdef_search_ctx->nvfb;
   const int nhfb = cdef_search_ctx->nhfb;
   cdef_search_ctx->sb_index =
-      aom_malloc(nvfb * nhfb * sizeof(cdef_search_ctx->sb_index));
+      aom_malloc(nvfb * nhfb * sizeof(cdef_search_ctx->sb_index[0]));
   cdef_search_ctx->sb_count = 0;
   cdef_search_ctx->mse[0] =
       aom_malloc(sizeof(**cdef_search_ctx->mse) * nvfb * nhfb);