8079841: Buffer underflow with empty zip entry names
Summary: check for zero-length entry name before accessing "last" byte
Reviewed-by: sherman, martin
length = 3i caught an error1,2,34length = 3length = 3I caught an error1,2,length = 3I caught an error1,2,length = 3I caught an error1,2,length = 3I caught an error1,2,length = 3I caught an error1,2,1,2,undefinedlength = 3