dc: unbreak interactive mode - was trying to get next tokens instead of executing

dc: unbreak interactive mode - was trying to get next tokens instead of executing

function                                             old     new   delta
zbc_program_read                                       -     268    +268
zdc_program_printStream                                -     146    +146
zbc_program_exec                                    4046    4182    +136
zdc_program_execStr                                  472     512     +40
zdc_parse_exprs_until_eof                              -      26     +26
zbc_vm_process                                       740     765     +25
zbc_lex_next                                        2225    2240     +15
zdc_parse_expr                                       569     535     -34
zbc_program_pushArray                                147       -    -147
zdc_program_asciify                                  370       -    -370
------------------------------------------------------------------------------
(add/remove: 3/2 grow/shrink: 4/1 up/down: 656/-551)          Total: 105 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>