Update SSToolkit/NSMutableArray+SSToolkitAdditions.m
Update SSToolkit/NSMutableArray+SSToolkitAdditions.m

Declaring i as an NSUInteger causes an exception when the NSMutable is empty. Indeed 0 - 1 == maximum integer value when the given integer is unsigned.
lijie121210/sstoolkit,carabina/sstoolkit,zjh171/sstoolkit,k131050059/sstoolkit,rtsbtx/sstoolkit,Amnysia/SSToolkit,zilaiyedaren/sstoolkit,andaluz/sstoolkit,TDness/sstoolkit,SuPair/sstoolkit,soffes/sstoolkit,Amnysia/SSToolkit,sideci-sample/sideci-sample-sstoolkit