[Bug-apl] Segfault in CDR parser

[Kacper Gutowski]

Hi,
Giving empty vector to 12⎕CR causes segmentation fault:

      12 ⎕CR ''


====================================================
SEGMENTATION FAULT

----------------------------------------
-- Stack trace at main.cc:88
----------------------------------------
0x7FE24301409B __libc_start_main
0x55F24EA68695  main
0x55F24EBB6A25   Workspace::immediate_execution(bool)
0x55F24EAB5541    Command::process_line()
0x55F24EAB5F55     Command::do_APL_expression(UCS_string&)
0x55F24EAB55DF      Command::finish_context()
0x55F24EABDE47       Executable::execute_body() const
0x55F24EB5D390        StateIndicator::run()
0x55F24EAF04D4         Prefix::reduce_statements()
0x55F24EAE9EB8          Prefix::reduce_A_F_B_()
0x55F24EB1F835           Quad_CR::eval_AB(Value_P, Value_P)
0x55F24EB1F50B            Quad_CR::do_CR(long, Value const*, PrintContext)
0x55F24EB1BF8B             Quad_CR::do_CR12(Value const&)
0x55F24EA9AA28              CDR::from_CDR(CDR_string const&, char const*)
0x7FE2435AD730               
0x55F24EA6F17B                
========================================
====================================================

Goodbye.
Session duration: 0.97774 seconds