Fix dividing by zero crashes of texture modifiers

[cx384]

• This fixes client crash with invalid verticalframe texture modifier #14223
• I also found a similar problem with ^[sheet which crashes when the width or height is zero, so this also gets fixed.
• Moreover, I removed some tailoring whitespaces.
• Imo the whole texture modifiers code needs to get refactored, but to be easier mergeable this PR only removes bugs that cause crashes. To name some further things that should be changed, unnecessary code duplication in the error code, many casts from int to u32 when using stoi (this causes unexpected integer overflows when using negative numbers) or some checks if the values are valid, e.g. whether frame_count > frame_index.

To do

This PR is Ready for Review.

How to test

Test if it does not crash anymore for verticalframe or sheet.

minetest.register_chatcommand("crashme", {
    func = function(name)
        minetest.show_formspec(name, "vframe:crashme", [[
            size[1,1]
            bgcolor[#FFFFFF]
            image[0,0;1,1;crack_anylength.png^\[verticalframe:0:0]
            image[0,0;1,1;crack_anylength.png^\[sheet:0x0:1,1]
        ]])
    end,
})

[SmallJoker]

Works on my PC.

[sfan5]

lgtm