Explicitly cast |CHECK| result to |void| to avoid compiler complaint.

Change-Id: Ie7b376da1a157d144e9c4651722630dbf9c45436
Reviewed-on: https://boringssl-review.googlesource.com/5021
Reviewed-by: Adam Langley <agl@google.com>

crypto/digest/digests.c

@@ -67,7 +67,7 @@
 #include "internal.h"
 
 #if defined(NDEBUG)
-#define CHECK(x) x
+#define CHECK(x) (void) (x)
 #else
 #define CHECK(x) assert(x)
 #endif