Suppress coverity warning about overflowing in safe_mem_is_zero

The unsigned underflow here is defined and intentional.

CID 202482

src/common/di_ops.c

@@ -218,6 +218,7 @@ safe_mem_is_zero(const void *mem, size_t sz)
     total |= *ptr++;
   }
 
+  /*coverity[overflow]*/
   return 1 & ((total - 1) >> 8);
 }