bc: use '\0' insteads of 0xff (BC_PARSE_STREND) as name terminator

bc: use '\0' insteads of 0xff (BC_PARSE_STREND) as name terminator

function                                             old     new   delta
zdc_program_printStream                                -     146    +146
zbc_program_exec                                    4003    4016     +13
zdc_parse_expr                                       473     470      -3
bc_parse_pushName                                     31      20     -11
bc_program_name                                       63      34     -29
zbc_program_pushArray                                147       -    -147
------------------------------------------------------------------------------
(add/remove: 1/1 grow/shrink: 1/3 up/down: 159/-190)          Total: -31 bytes

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