[616] Fix for abort on action button create with a zero length string
If the buffer limit is zero size the action button's text member is pointed at an empty string, which is a pointer to a constant within the module, then an unconditional call is made to string_copy() the gadget template text into that member. In the case of a ROM copy of Window this results in an abort.
Adopt the same strategy as the other gadgets (eg. the radio button gadget) and use string_copy_chk() instead - this is a noop for buffer limits of zero size.