Execute on empty element produces a stack trace

[Hans-Peter Sorge]

    Hi, 
    
      This gives a stack trace:
                  X←0 2⍴ ⊂'0'
                  ⍎¨X[;1]
      
==============================================================================
      Assertion failed: start <= body_from_to.low
      in Function:      set_error_info
      in file:          Executable.cc:602
      
      C/C++ call stack:
      *** useless apl.lines (no CXXFLAGS=-rdynamic -gdwarf-2)
      
      ----------------------------------------
      -- Stack trace at Assert.cc:75
      ----------------------------------------
      0x7f7221849c4b __libc_start_main
      0x7f7221849b8a  
      0x4d3f27   main
      0x69eacb    Workspace::immediate_execution(bool)
      0x52e649     Command::process_line()
      0x52e8a4      Command::process_line(UCS_string&,
      std::ostream*)
      0x530ebc       Command::do_APL_expression(UCS_string&)
      0x5311b0        Command::finish_context()
      0x55588d         Executable::execute_body() const
      0x638d2e          StateIndicator::run()
      0x590e0a           Prefix::reduce_statements()
      0x591d55            Prefix::reduce_MISC_F_B_()
      0x550095             DerivedFunction::eval_B(Value_P) const
      0x5160e7              Bif_OPER1_EACH::eval_LB(Token&, Value_P)
      const
      0x515232               Bif_OPER1_EACH::do_eval_LB(Token&,
      Value_P)
      0x4ec3d3                Token::get_apl_val() const
      0x553a38                 throw_apl_error(ErrorCode, char const*)
      0x5540a1                 
      Error::update_error_info(StateIndicator*)
      0x555fd0                   Executable::set_error_info(Error&,
      Function_PC2) const
      0x4f282e                    do_Assert(char const*, char const*,
      char const*, int)
      ========================================
      
      SI stack:
      
      Depth:      1
      Exec:       0xabb4a0
      Safe exec:  0
      Pmode:      ⍎  
      PC:         0 (1) RETURN_EXEC
      Stat:       
      err_code:   0x0
      
      Depth:      0
      Exec:       0xacf8b0
      Safe exec:  0
      Pmode:      ◊  ⍎¨X[;1]
      PC:         8 (9) RETURN_STATS
      Stat:       ⍎¨X[;1]
      err_code:   0x0
      
      
==============================================================================
      
      
      Best Regards
      Hans-Peter