chain: make some integer conversions explicit

We have an unsigned constant which we're bit-inverting, or'ing into a
signed constant, then assigning back to the signed constant. We should
make it explicit wth is going on here.

Author: apoelstra

src/chain.h

@@ -469,7 +469,7 @@ class CDiskBlockIndex : public CBlockIndex
     bool RemoveDynaFedMaskOnSerialize(bool for_read) {
         if (for_read) {
             bool is_dyna = nVersion < 0;
-            nVersion = ~CBlockHeader::DYNAFED_HF_MASK & nVersion;
+            nVersion = (int32_t) (~CBlockHeader::DYNAFED_HF_MASK & (uint32_t)nVersion);
             return is_dyna;
         } else {
             return is_dynafed_block();