Added condition to check if HEX code contain only valid characters (issue #1015)

[peruginni]

Hello,

I have fixed issue #1015. Less was recognizing HEX code of length 3 inside intended longer HEX code, where were invalid hexadecimal characters.

This is my first contribution to less.js, so if there is something I missed - please tell me, I will gladly improve it. Have a nice day,

Ondra

[seven-phases-max]

I have fixed the similar problems of color function in 3322609. Probably we should consider to re-use the same code somehow as there're more issues than just "incorrect characters". See [6] examples in #1604.

[lukeapage]

I'm not sure how they could be merged. I would only make the error less specific and remove the length check.. e.g. this would catch

#fffblack

Where they have missed a space.

[peruginni]

Thank you for answers. Tomorrow I will fix it to cover missed space case and add better error text.

[lukeapage]

great thanks.