Prevent hang when the point is at the bottom of the buffer

[danielmartin]

Fixes #157

If the point is at the bottom of the buffer and which-function-mode is enabled, Emacs freezes. The reason is that the code enters the "unmatched parens" part of the logic and gets stuck navigating blocks forward and backward without ever matching a block correctly.

This can be fixed if we check for the end of buffer case explicitly and return a whole buffer as a block. This makes a point "in between functions" return the same function information as a point at the end of the buffer. Graphically:

protocol P { func f() }
extension P { func g() { f() } }
struct X<T> : P {
    func f() { print("unconstrained") }
}
<--- Point here (which-function-mode returns "P")
struct X<T> : where T : BinaryInteger {
    func f() { print("constrained")
}
<--- Point here (which-function-mode returns "P")

[taku0]

Thank you for your PR. I will test this in the next weekend.

[taku0]

It seems that swift-mode:beginning-of-defun gets stuck at the beginning of buffer. I will investigate this further tomorrow.

[taku0]

Fixed by 04c8a621e9844ef3f7d53c41b046f71aadb1a9d2. Thank you for your PR.