Fix exception thrown when there is no facebook system account

The fix derived from this crash report (using 3.5.2)
Fatal Exception NSRangeException
*** -[__NSArrayI objectAtIndex:]: index 0 beyond bounds for empty array
0	 CoreFoundation	 __exceptionPreprocess + 162
1	 libobjc.A.dylib	 objc_exception_throw + 30
2	 CoreFoundation	 -[__NSArrayI objectAtIndex:] + 164
3	 StatusShuffle 	
FBSystemAccountStoreAdapter.m line 205
__119-[FBSystemAccountStoreAdapter requestAccessToFacebookAccountStore:defaultAudience:isReauthorize:appID:session:handler:]_block_invoke_3
4	 libdispatch.dylib	 _dispatch_call_block_and_release + 10
5	 libdispatch.dylib	 _dispatch_client_callout + 22
6	 libdispatch.dylib	 _dispatch_main_queue_callback_4CF$VARIANT$up + 226
7	 CoreFoundation	 __CFRunLoopRun + 1290
8	 CoreFoundation	 CFRunLoopRunSpecific + 356
9	 CoreFoundation	 CFRunLoopRunInMode + 104
10	 GraphicsServices	 GSEventRunModal + 74
11	 UIKit	 UIApplicationMain + 1120