bench: fixed ubsan implicit conversion

The benchmarks can now run much longer due to the minimum of 10ms or
directly with -min_time. With -min_time=20000 I could trigger two ubsan
errors in the benchmarks, which are fixed in this commit by using
unsigned type and adding "& 0xFF".

src/bench/crypto_hash.cpp

@@ -110,9 +110,9 @@ static void MuHash(benchmark::Bench& bench)
 {
     MuHash3072 acc;
     unsigned char key[32] = {0};
-    int i = 0;
+    uint32_t i = 0;
     bench.run([&] {
-        key[0] = ++i;
+        key[0] = ++i & 0xFF;
         acc *= MuHash3072(key);
     });
 }

src/bench/rollingbloom.cpp

@@ -13,16 +13,16 @@ static void RollingBloom(benchmark::Bench& bench)
     uint32_t count = 0;
     bench.run([&] {
         count++;
-        data[0] = count;
-        data[1] = count >> 8;
-        data[2] = count >> 16;
-        data[3] = count >> 24;
+        data[0] = count & 0xFF;
+        data[1] = (count >> 8) & 0xFF;
+        data[2] = (count >> 16) & 0xFF;
+        data[3] = (count >> 24) & 0xFF;
         filter.insert(data);
 
-        data[0] = count >> 24;
-        data[1] = count >> 16;
-        data[2] = count >> 8;
-        data[3] = count;
+        data[0] = (count >> 24) & 0xFF;
+        data[1] = (count >> 16) & 0xFF;
+        data[2] = (count >> 8) & 0xFF;
+        data[3] = count & 0xFF;
         filter.contains(data);
     });
 }