bc: avoid having to twiddle b->neg in zbc_num_p()

bc: avoid having to twiddle b->neg in zbc_num_p()

function                                             old     new   delta
zbc_num_ulong_abs                                      -      70     +70
zbc_num_p                                            424     413     -11
zbc_num_ulong                                         81      21     -60
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(add/remove: 1/0 grow/shrink: 0/2 up/down: 70/-71)             Total: -1 bytes

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