bc: fix infinite state growth for "yes 1 | bc" case

bc: fix infinite state growth for "yes 1 | bc" case

function                                             old     new   delta
zbc_vm_process                                       585     672     +87
bc_func_init                                          50      86     +36
zbc_program_num                                      990    1022     +32
bc_program_str                                        17      47     +30
bc_program_current_func                                -      22     +22
bc_parse_pushNUM                                      66      87     +21
bc_func_free                                          27      43     +16
zbc_num_binary                                       145     147      +2
bc_program_reset                                      64      61      -3
bc_parse_pushSTR                                      73      65      -8
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 7/2 up/down: 246/-11)           Total: 235 bytes
   text	   data	    bss	    dec	    hex	filename
 981393	    485	   7296	 989174	  f17f6	busybox_old
 981656	    485	   7296	 989437	  f18fd	busybox_unstripped

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