target-bsnes: Fix crash when clicking in an empty row while assiging inputs.

When assigning an input, you can click on a different row of the bindings table
to change which input you're assigning. As well as listening for keyboard and
controller input, bsnes displays buttons to map the mouse axes or mouse buttons,
depending on whether the new row you've selected represents an analog or digital
input.

However, some controllers do not have enough inputs to fill the default size of
the input table. If you click below the last row, then *no* input is selected,
and when bsnes tries to find the input associated with that not-a-row, it
crashes.

The solution is, only show the mouse axis/mouse button buttons when a valid row
is selected.

Mentioned in #75.

bsnes/target-bsnes/settings/input.cpp

@@ -67,7 +67,7 @@ auto InputSettings::updateControls() -> void {
   assignMouse2.setVisible(false);
   assignMouse3.setVisible(false);
 
-  if(activeMapping) {
+  if(activeMapping && batched) {
     auto& input = activeDevice().mappings[batched.left().offset()];
     if(input.isDigital()) {
       assignMouse1.setVisible().setText("Mouse Left");