Add sanity check for SPI dataBitLength

[AdrianSoundy]

Description

Add sanity check to dataBitLength.
If zero seems to dump out memory.

Fixes nanoframework/Home#497

How Has This Been Tested?

Run provided test code.

Types of changes

• Improvement (non-breaking change that improves a feature, code or algorithm)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Signed-off-by: adriansoundy adriansoundy@gmail.com

[nfbot]

Hi @AdrianSoundy,

I'm nanoFramework bot.
Thank you for your contribution!

A human will be reviewing it shortly. 😉

[josesimoes]

@AdrianSoundy wouldn't be better to throw a bad argument exception instead of "fixing" the value?
I mean, as it is it works, of course, but the developer is not aware that he has made a mistake or set it to a wrong value.

That's the usual procedure throughout the code base and it's done in a number of places.

From what I could found (I've only worked with 8 and 16 bits) but the data can be from 8 to 16. I don't know if ESP32 can handle anything in the between...

So I suggest that the check is made for a valid number >= 8 and <= 16 (or just 8 and 16 to keep it simple) and throw an exception on invalid argument. What do you think?