bitstream: fix logical-not-parentheses warning

Change-Id: I8b70f9251a23707f2ef83889403d40a7796533a7

vp10/encoder/bitstream.c

diff --git a/vp10/encoder/bitstream.c b/vp10/encoder/bitstream.c
index 5bf9f98..0b26458 100644
--- a/vp10/encoder/bitstream.c
+++ b/vp10/encoder/bitstream.c
@@ -312,7 +312,7 @@
     if (cm->reference_mode == REFERENCE_MODE_SELECT) {
       vpx_write(w, is_compound, vp10_get_reference_mode_prob(cm, xd));
     } else {
-      assert(!is_compound == (cm->reference_mode == SINGLE_REFERENCE));
+      assert((!is_compound) == (cm->reference_mode == SINGLE_REFERENCE));
     }
 
     if (is_compound) {