chroma_check: don't access UV planes if monochrome

In chroma_check(), return immediately if we are generating a monochrome
encoded stream. This avoids accessing the U, V planes when they are
NULL.

BUG=aomedia:2723

Change-Id: I26a39791f820b4d4e1d63ff7141f594c3c7181f5
(cherry picked from commit 098327e0a20fbbcf8e942261f43b30be8475f903)

av1/encoder/var_based_part.c

diff --git a/av1/encoder/var_based_part.c b/av1/encoder/var_based_part.c
index 504833e..e3cb1fa 100644
--- a/av1/encoder/var_based_part.c
+++ b/av1/encoder/var_based_part.c
@@ -581,7 +581,7 @@
   int i;
   MACROBLOCKD *xd = &x->e_mbd;
 
-  if (is_key_frame) return;
+  if (is_key_frame || cpi->oxcf.monochrome) return;
 
   for (i = 1; i <= 2; ++i) {
     unsigned int uv_sad = UINT_MAX;