bc: add code to detect errors like "print 1 print 2"

bc: add code to detect errors like "print 1 print 2"

function                                             old     new   delta
zbc_vm_process                                       831     925     +94
zbc_program_exec                                    3964    3976     +12
zdc_program_execStr                                  506     512      +6
zbc_lex_next                                        2161    2167      +6
zbc_program_assign                                   419     424      +5
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(add/remove: 0/0 grow/shrink: 5/0 up/down: 123/0)             Total: 123 bytes

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