Luis Javier Merino authored Jun 22, 2021 and Tomaz Canabrava committed Jun 24, 2021

When the last line in history was marked as wrapped, the history reflow
algorithm could try to make an out-of-bound access.

These were found via repeteadly zooming-in and out in the amp.rs editor
(available through Rust's cargo tool).  amp.rs writes in the normal
buffer (not in the alternate buffer).

While at it, add a Q_ASSERT in HistoryScrollFile::startOfLine() to
ensure parameters are in range.  Note that after an addCells()/addLine()
pair, HistoryScrollFile::startOfLine(getLines) always has the same value
as _cells.len().

BUG: 436916