Fixes accessing non-existant form on game exit.

gameplay/src/Form.cpp

@@ -657,8 +657,7 @@ bool Form::mouseEventInternal(Mouse::MouseEvent evt, int x, int y, int wheelDelt
 {
     bool eventConsumed = false;
 
-    size_t size = __forms.size();
-    for (size_t i = 0; i < size; ++i)
+    for (size_t i = 0; i < __forms.size(); ++i)
     {
         Form* form = __forms[i];
         GP_ASSERT(form);