Fix msvc compiler error C4146 (minus operator applied to unsigned type)

On msvc14, the compiler error C4146 (unary minus operator applied to unsigned type, result still unsigned) had been occured.
Use '0 - x' styled formula instead of '-x' so as to fix the error.

src/bloom.cpp

@@ -254,8 +254,8 @@ void CRollingBloomFilter::insert(const std::vector<unsigned char>& vKey)
         if (nGeneration == 4) {
             nGeneration = 1;
         }
-        uint64_t nGenerationMask1 = -(uint64_t)(nGeneration & 1);
+        uint64_t nGenerationMask1 = 0 - (uint64_t)(nGeneration & 1);
-        uint64_t nGenerationMask2 = -(uint64_t)(nGeneration >> 1);
+        uint64_t nGenerationMask2 = 0 - (uint64_t)(nGeneration >> 1);
         /* Wipe old entries that used this generation number. */
         for (uint32_t p = 0; p < data.size(); p += 2) {
             uint64_t p1 = data[p], p2 = data[p + 1];