Avoid possible divide by zero

On the off chance that in some configurations the terminal width is set
to zero, avoid an exception by returning 0 for terminal lines. It is
likely that supershell will not work well if terminal width is zero, but
that's better than a potential crash (I think the crash would be in the
progress background thread, so I'm not sure how bad it would be, but
still its good to avoid).

internal/util-logging/src/main/scala/sbt/internal/util/ConsoleAppender.scala

@@ -389,7 +389,7 @@ class ConsoleAppender private[ConsoleAppender] (
     out.flush()
   }
   private def terminalLines(width: Int): String => Int =
-    (progressLine: String) => (progressLine.length - 1) / width
+    (progressLine: String) => if (width > 0) (progressLine.length - 1) / width else 0
   private def deleteConsoleLines(n: Int): Unit = {
     (1 to n) foreach { _ =>
       out.println(DeleteLine)