Transaction: Fix bounds error getting data address in empty seq

In the unusual case where log data is zero-length, `data[0].addr` is invalid
and Nim thoughtfully raises `IndexOutOfBounds`, a `Defect` so it's not even
in `CatchableError`.

This is done in the EVMC host services to handle `LOG*` ops, and it made one
of the EVM tests silently fail with no error message.  The fix is obvious.

Signed-off-by: Jamie Lokier <jamie@shareable.org>

nimbus/transaction/host_services.nim

@@ -212,8 +212,10 @@ proc emitLog(host: TransactionHost, address: HostAddress,
     for i in 0 ..< count:
       log.topics[i] = topicsArray[i]
 
-  log.data = newSeq[byte](data_size.int)
-  copyMem(log.data[0].addr, data, data_size.int)
+  if (data_size > 0):
+    log.data = newSeq[byte](data_size.int)
+    copyMem(log.data[0].addr, data, data_size.int)
+
   log.address = address
   host.logEntries.add(log)