Replace usage of ArrayIndexOutOfBoundsException in treewalk

Replace usage of ArrayIndexOutOfBoundsException in treewalk

Using exceptions during normal operations - for example with the
desire of expanding an array in the failure case - can have a
severe performance impact. When exceptions are instantiated,
a stack trace is collected. Generating stack trace can be expensive.

Compared to that, checking an array for length - even if done many
times - is cheap since this is a check that can run in just a
handful of CPU cycles.

Change-Id: Ifaf10623f6a876c9faecfa44654c9296315adfcb
Signed-off-by: Patrick Hiesel <hiesel@google.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>