bc: speed up string printing, fix print ""

bc: speed up string printing, fix print ""

function                                             old     new   delta
static.esc                                             -       9      +9
zxc_program_print                                    681     683      +2
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 1/0 up/down: 11/0)               Total: 11 bytes
   text	   data	    bss	    dec	    hex	filename
 979144	    485	   7296	 986925	  f0f2d	busybox_old
 979062	    485	   7296	 986843	  f0edb	busybox_unstripped

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