[Bug-apl] 19⎕CR prints stack trace for bad UTF-8

[Kacper Gutowski]

Giving malformed string to 19⎕CR results in it printing an error
message and a stack trace, and then returning an empty vector.  As
it's generally very useful to have generic routine parsing UTF-8
data at hand, I think it shouldn't do that.  If it's an error, it
should be possible to handle it with ⎕EC or ⎕EA.  I would expect
it to return a domain error; after all, malformed vector is simply
outside the domain of valid UTF-8 vectors.  Although I'm not
against it returning empty vector as it's easier to handle, it
just shouldn't print that stack trace.


      8⎕CR⎕EC'19⎕CR ⎕UCS 128'
Bad UTF8 string: 0x80 at UCS_string.cc:65

----------------------------------------
-- Stack trace at UCS_string.cc:66
----------------------------------------
0x7f4810a37610 __libc_start_main
0x443db5  main
0x579b9d   Workspace::immediate_execution(bool)
0x48b22a    Command::process_line()
0x48b2ea     Command::do_APL_expression(UCS_string&)
0x496135      Executable::execute_body() const
0x525370       StateIndicator::run()
0x4cb918        Prefix::reduce_statements()
0x4c6ae5         Prefix::reduce_A_F_B_()
0x4fe154          Quad_CR::eval_AB(Value_P, Value_P)
0x4fc3b5           Quad_CR::do_CR(long, Value const&, PrintContext)
0x556e26            UCS_string::UCS_string(UTF8_string const&)
========================================
┌→──────────┐
│1 ┌→──┐ ┌⊖┐│
│  │0 0│ │ ││
│  └───┘ └─┘│
└∊──────────┘


-k