[Bug-apl] Segfault when tracing not a function

[Kacper Gutowski]

Using ⎕TRACE or ⎕STOP with some other symbol than user defined
function either through T∆/S∆ or directly, either for querying or
setting, results in a segmentation fault:

      A←3
      ⎕TRACE'A'


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

----------------------------------------
-- Stack trace at main.cc:63
----------------------------------------
0x7f6fa8a8ab45 __libc_start_main
0x443325  main
0x56e57d   Workspace::immediate_execution(bool)
0x48a45a    Command::process_line()
0x48a51a     Command::do_APL_expression(UCS_string&)
0x4953c5      Executable::execute_body() const
0x51bb80       StateIndicator::run()
0x4c9c58        Prefix::reduce_statements()
0x4c8f3e         Prefix::reduce_MISC_F_B_()
0x4ea8d7          Quad_TRACE::eval_B(Value_P)
0x4ea4f6           Stop_Trace::locate_fun(Value const&)
0x7f6fa98cb8d0
0x4482d7
========================================
====================================================

Goodbye.


-k