Don't crash dropdown menu when selectedIndex is less than 0

[jeroencoumans]

Fixes 'Uncaught TypeError: Cannot read property 'text' of undefined' when passing an index of -1.

[oliviertassinari]

I would rather right

var selectedIndex = this.state.selectedIndex;
selectedIndex = (selectedIndex > -1) ? selectedIndex : 0;

for a better optimisation with uglifyjs

[mmrtnz]

Can you include a console warning when users pass a negative index? Negative indices are typically due to poorly written code or bugs. A warning will notify users of the potential error. I would also follow @oliviertassinari's suggestion.

Please follow these examples when writing the warning.

This PR will be good to go after those two changes are made.

[jeroencoumans]

I've changed it according to your comments. I also moved the check to componentWillReceiveProps, which seems more appropriate than doing this in render.

[oliviertassinari]

Be aware that componentWillReceiveProps is not called for the initial render.

[jeroencoumans]

I've made a separate method that's called on mounting and when receiving props. Merged and pulled and rebased against master... let me know if this is fine :)

[mmrtnz]

Thanks @jeroencoumans!