bc: make bc_vm_envArgs() NULL-terminate its argv, simplify bc_vm_envLen()

bc: make bc_vm_envArgs() NULL-terminate its argv, simplify bc_vm_envLen()

function                                             old     new   delta
bc_num_ulong                                          92     103     +11
static.nullptr                                         -       4      +4
bc_args                                              120     123      +3
static.bc_args_env_name                                4       -      -4
dc_main                                               49      41      -8
bc_main                                               49      41      -8
bc_vm_run                                           1917    1883     -34
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(add/remove: 1/1 grow/shrink: 2/3 up/down: 18/-54)            Total: -36 bytes

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