display: do not attempt to draw a line that is outside the viewport

When we're on the last row in the viewport, it is unwise to try and draw
the next line.

This fixes https://savannah.gnu.org/bugs/?65539.
The issue was reported by `correctmost`.

Bug existed since commit c2322f85 from three days ago.

src/nano.c

@@ -1522,7 +1522,8 @@ void inject(char *burst, size_t count)
 #else
 		if (margin)
 #endif
-			update_line(thisline->next, 0);
+			if (openfile->current_y < editwinrows - 1)
+				update_line(thisline->next, 0);
 	}
 
 #ifndef NANO_TINY