[Bug-apl] Segfault in outer product with empty argument

[Kacper Gutowski]

Outer product of any function with empty left or right argument,
results in segmentation fault:


      'abc' ∘.= ''


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

----------------------------------------
-- Stack trace at main.cc:122
----------------------------------------
0x7f90f668cb45 __libc_start_main
0x435995  main
0x529edd   Workspace::immediate_execution(bool)
0x4651cd    Command::process_line()
0x463edd     Command::process_line(UCS_string&)
0x46e678      Executable::execute_body() const
0x4e4370       StateIndicator::run()
0x4912ca        Prefix::reduce_statements()
0x48e293         Prefix::reduce_A_F_B_()
0x4e514a          StateIndicator::eval_AB(Token&, Token&, Token&)
0x476173           DerivedFunction::eval_AB(Value_P, Value_P)
0x44df1d            Bif_OPER2_PRODUCT::eval_ALRB(Value_P, Token&, Token&, 
Value_P)
0x44b4ad             Bif_OPER2_PRODUCT::outer_product(Value_P, Token&, Value_P)
0x44ad8a              Bif_OPER2_PRODUCT::finish_outer_product(EOC_arg&)
0x45b9a4               Cell::init_from_value(Value_P, char const*)
0x45b7e9                Cell::init(Cell const&)
0x7f90f85e3890                 
0x47e22a                  
========================================
====================================================



-k