fix: prevent integer underflow in amount() bounds check

The bounds check 'index > arr.size() - 1' has an edge case bug.
When arr.size() is 0, subtracting 1 from an unsigned size_t
underflows to SIZE_MAX, so 'index > SIZE_MAX' is always false.

This could theoretically allow out-of-bounds access, though it's
pretty hard to trigger in practice - would need a malformed/corrupted
unsigned_tx file that parses successfully but has no transactions.

Changed to 'arr.empty() || index >= arr.size()' which handles
the edge case properly.

Found with AddressSanitizer during fuzzing.

src/libwalletqt/UnsignedTransaction.cpp

@@ -19,7 +19,7 @@ QString UnsignedTransaction::errorString() const
 quint64 UnsignedTransaction::amount(size_t index) const
 {
     std::vector<uint64_t> arr = m_pimpl->amount();
-    if(index > arr.size() - 1)
+    if(arr.empty() || index >= arr.size())
         return 0;
     return arr[index];
 }