bc: bc_num_ulong2num(), bc_program_pushGlobal(), bc_program_stackLen() never fail

bc: bc_num_ulong2num(), bc_program_pushGlobal(), bc_program_stackLen() never fail

function                                             old     new   delta
bc_num_ulong2num                                      66      62      -4
bc_num_printNum                                      572     563      -9
bc_program_exec                                     4562    4544     -18
bc_program_num                                      1147    1126     -21
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/4 up/down: 0/-52)             Total: -52 bytes
   text	   data	    bss	    dec	    hex	filename
 988575	    485	   7296	 996356	  f3404	busybox_old
 988523	    485	   7296	 996304	  f33d0	busybox_unstripped

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