wallet2: initialize amount to 0 in tx_scan_info_t ctor

It seems the more prudent thing to do here. It will not catch
attempts to use that value before it is initialized when using
ASAN or valgrind, but in a case where it does, it will have
smaller repercussions.

So it seems appropriate in this particular case.

Coverity 182498

src/wallet/wallet2.h

@@ -229,7 +229,7 @@ namespace tools
       bool error;
       boost::optional<cryptonote::subaddress_receive_info> received;
 
-      tx_scan_info_t(): money_transfered(0), error(true) {}
+      tx_scan_info_t(): amount(0), money_transfered(0), error(true) {}
     };
 
     struct transfer_details