When the user entered a driver expression like `#frame` into a number
field, Blender would crash sometime after. This is because
e1a9ba94c5 did not handle this case properly and ignored the fact
that the user can enter text into the field.
The fix makes sure that we only cancel if the value is a *number* equal
to the previous value in the field.
Note that this could also be handled by `ui_but_string_set` which would
be a bit nicer potentially. However, I was unable to find a clean way of
passing `data->startvalue` into that function. `uiHandleButtonData` is
only known locally inside `interface_handlers.c`.
Reviewed By: Severin
Maniphest Tasks: T87688
Differential Revision: https://developer.blender.org/D11049
bbb52a462e