bc: tighten up input NUL handling

bc: tighten up input NUL handling

function                                             old     new   delta
static.dc_lex_tokens                                   -      90     +90
bc_error_bad_character                                17      31     +14
static.dc_lex_regs                                     -      13     +13
bc_read_line                                         406     410      +4
bc_program_index                                      64      66      +2
dc_lex_regs                                           13       -     -13
zdc_parse_expr                                       671     656     -15
zbc_lex_next                                        2318    2230     -88
dc_lex_tokens                                         91       -     -91
------------------------------------------------------------------------------
(add/remove: 2/2 grow/shrink: 3/2 up/down: 123/-207)          Total: -84 bytes
   text	   data	    bss	    dec	    hex	filename
 981667	    485	   7296	 989448	  f1908	busybox_old
 981599	    485	   7296	 989380	  f18c4	busybox_unstripped

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