Use Custom Storage dialog resets argument type

[lab313ru]

Subj. To reproduce:

1. Check "Use Custom Storage" checkbox
2. Add a new argument
3. Double click at the new argument to show the dialog
4. Enter any known data type, for example dword
5. Add a register to store the type
6. Pressing OK will reset the data type to the corresponding undefinedX type. In this case it will be undefined4.