NSString._getBlockStart - loose precondition check to avoid crash in edge case

[lxbndr]

I believe it was just a typo. There are other similar checks already, and all of them use <= comparison.

This was discovered as the reason of paragraphRange(for:) fail on whole string range. E.g, following code is crashing:

let s = "abc" as NSString
let range = NSRange(location: 0, length:3)
_ = s.paragraphRange(for: range) // <- crash

Test is also included.

Test checks paragraphRange on:

• whole string range
• every subrange in every single paragraph
• every subrange in every two consecutive paragraphs

[spevans]

@swift-ci test

[compnerd]

This does seem like a typo. Thanks for fixing this.