ACPICA: utstring: Check array index bound before use.

ACPICA BZ 1077. David Binderman.

References: https://bugs.acpica.org/show_bug.cgi?id=1077


Signed-off-by: David Binderman <dcb314@hotmail.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Cc: 3.9+ <stable@vger.kernel.org> # 3.9.x: 42f8fb75: ACPICA: Source restructuring
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>