bc: simplify, stop testing for IO errors on every output

bc: simplify, stop testing for IO errors on every output

function                                             old     new   delta
bc_read_line                                         297     342     +45
bc_vm_run                                           2601    2608      +7
bc_program_reset                                     178     182      +4
bc_args                                              125     123      -2
dc_name                                                3       -      -3
bc_name                                                3       -      -3
bc_program_printStream                               176     172      -4
bcg                                                   48      40      -8
dc_main                                               97      80     -17
bc_main                                               97      80     -17
bc_vm_fflush                                          21       -     -21
bc_vm_puts                                            23       -     -23
bc_vm_printf                                          36       -     -36
bc_vm_putchar                                         46       -     -46
bc_vm_exit                                            46       -     -46
bc_vm_process                                        361     312     -49
bc_vm_info                                            76      19     -57
bc_copyright                                         155       -    -155
------------------------------------------------------------------------------
(add/remove: 0/8 grow/shrink: 3/7 up/down: 56/-487)          Total: -431 bytes
   text	   data	    bss	    dec	    hex	filename
 989722	    485	   7344	 997551	  f38af	busybox_old
 989491	    485	   7336	 997312	  f37c0	busybox_unstripped

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