Use uint64_t instead of size_t for serialized indices into tx.vjoinsplit

size_t has no guarantee of uniform size, and thus far has been serialized as a
uint64_t on supported platforms.

src/wallet/wallet.h

@@ -159,7 +159,7 @@ public:
     // Transaction hash
     uint256 hash;
     // Index into CTransaction.vjoinsplit
-    size_t js;
+    uint64_t js;
     // Index into JSDescription fields of length ZC_NUM_JS_OUTPUTS
     uint8_t n;