fix the OOB buffer access issue in src/text/draw.rs when switching to text view at the end of the file

[yeggor]

If you open a file larger than area.height * area.width and go to the last page (G), then switch to text view (Enter), you should get a similar panic:

> dz6 rand.bin

thread 'main' (27108978) panicked at src/text/draw.rs:17:24:
range end index 30497841225 out of range for slice of length 30497832960
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

In this PR I am fixing the issue.

NOTE: should be merged on top of #39.

[merces]

Thanks a lot!