busybox

bc: do not allow "()" as a valid expression

function                                             old     new   delta
bc_parse_expr_empty_ok                                 -    2178   +2178
bc_parse_expr                                       2178      49   -2129
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(add/remove: 1/0 grow/shrink: 0/1 up/down: 2178/-2129)         Total: 49 bytes

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