Fixed the array access beyond bounds.

It has crashed for some projects with following stacktrace.

*********************
NSRangeException: *** -[__NSArrayI objectAtIndex:]: index 3 beyond bounds [0 .. 2]
  @ 0   CoreFoundation                      0x00007fff87bf8f56 __exceptionPreprocess + 198
  @ 1   libobjc.A.dylib                     0x00007fff8fac7dee objc_exception_throw + 43
  @ 2   CoreFoundation                      0x00007fff87ba2370 -[__NSArrayI objectAtIndex:] + 208
  @ 3   appledoc                            0x00000001053eff60 _mh_execute_header + 155488
...
*********************

So, I added the bounds checking.