vp8 onyx_if: assert divide by zero

Clears up static clang analysis warning regarding divide by zero.

Trying to explain to the compiler how it's impossible to avoid
incrementing num_blocks at least once is difficult.

Change-Id: Ibaae43be572e5cd7a689b440dcd341c17d33443b

vp8/encoder/onyx_if.c

@@ -43,6 +43,7 @@
 #endif
 #include "encodeframe.h"
 
+#include <assert.h>
 #include <math.h>
 #include <stdio.h>
 #include <limits.h>
@@ -3052,6 +3053,7 @@ static int measure_square_diff_partial(YV12_BUFFER_CONFIG *source,
   }
   // Only return non-zero if we have at least ~1/16 samples for estimate.
   if (num_blocks > (tot_num_blocks >> 4)) {
+    assert(num_blocks != 0);
     return (Total / num_blocks);
   } else {
     return 0;