simplewallet: forbid 0 ring size

It'd be interpreted as a huge one (~0 fake outs)

src/simplewallet/simplewallet.cpp

@@ -2585,6 +2585,11 @@ bool simple_wallet::transfer_main(int transfer_type, const std::vector<std::stri
       if (fake_outs_count == 0)
         fake_outs_count = DEFAULT_MIX;
     }
+    else if (ring_size == 0)
+    {
+      fail_msg_writer() << tr("Ring size must not be 0");
+      return true;
+    }
     else
     {
       fake_outs_count = ring_size - 1;
@@ -3196,6 +3201,11 @@ bool simple_wallet::sweep_main(uint64_t below, const std::vector<std::string> &a
       if (fake_outs_count == 0)
         fake_outs_count = DEFAULT_MIX;
     }
+    else if (ring_size == 0)
+    {
+      fail_msg_writer() << tr("Ring size must not be 0");
+      return true;
+    }
     else
     {
       fake_outs_count = ring_size - 1;