[terminal] IJPL-203233 Ensure anyLineChanged flag is set before flushing the changes

There are reports with exceptions where for some reason, `linesDiscardedFromHistory` is called without preceding `linesChanged`, so `anyLineChanged` flag is not set. As a result, we get NPE here and crush the terminal session emulation.
Since the actual case is unclear, let's just ensure that this flag is set.

GitOrigin-RevId: dcd27ea51fc1dbc69889fe2d9ff443139ea250dd

plugins/terminal/backend/src/com/intellij/terminal/backend/TerminalContentChangesTracker.kt

@@ -57,6 +57,7 @@ class TerminalContentChangesTracker(
         else {
           val additionalLines = lines.subList(lastChangedVisualLine, lines.size)
           lastChangedVisualLine = 0
+          anyLineChanged = true
           flushChanges(additionalLines)
         }
       }