Prevent a panic when uncommenting a line with only a comment token (#5933)

Open a new document `test.rs` and type the following:
`di//<esc><C-c>`

The margin calculation pushes the range out of bounds for the comment
marker when there are no characters (newline) after it.

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value:
Char range out of bounds: char range 0..3,
Rope/RopeSlice char length 2', ropey-1.6.0/src/rope.rs:546:37

The debug build catches the error in the transaction: thread 'main'
panicked at 'attempt to subtract with overflow',
helix-core/src/transaction.rs:503:26

0 files changed, 0 insertions, 0 deletions