TextInput: Fix for crashes caused by text selection outside of TextInput area

[RedGui]

This may be an edge case, but worth addressing.

Changes:

• Fix for uncaught exceptions due to text selection exceeding TextInput area

Description:

Fix for uncaught exceptions related to slicing when list indexes are not supplied as integers. This happens for row / col in _lines[row][:col] and other places that use slicing for cursor position or text selection, even double tap.

Observed in:

in KivyMD's class MDTextFieldRound when for example

1. Created password input field of type MDTextFieldRound with a right side icon (no left side icon) enabled in the text field

2. Populated the field with text

3. Resized (downsized) the window until the icon pushes the text beyond the text field area (overflow text is hidden underneath the visible layer)

4. Selected the text by clicking in a way that:

   • Targeted the beginning of the overflow or an invisible part of the text (obstructed by a part of the input field if it is rounded or partly rounded)
   • 'Double tapped' the text, selecting the entire text (overflow as well)

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[tshirtman]

Seems like it can't hurt, and if it prevents edge case failure, then sure…

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