Make CMutableTransaction constructor explicit

Silently converting to a CMutableTransaction will drop all caches
and should thus be done explicitly

src/wallet/feebumper.cpp

@@ -185,7 +185,7 @@ Result CreateTransaction(const CWallet* wallet, const uint256& txid, const CCoin
     // If the output is not large enough to pay the fee, fail.
     CAmount nDelta = new_fee - old_fee;
     assert(nDelta > 0);
-    mtx =  *wtx.tx;
+    mtx = CMutableTransaction{*wtx.tx};
     CTxOut* poutput = &(mtx.vout[nOutput]);
     if (poutput->nValue < nDelta) {
         errors.push_back("Change output is too small to bump the fee");