fbsplash: fix broken handling of buffered case:

fbsplash: fix broken handling of buffered case:
"{echo 45; echo 33; } | { sleep 1; fbsplash -f - ...; }"

function                                             old     new   delta
fb_drawprogressbar                                     -     413    +413
xmalloc_fgetline                                       -      46     +46
xmalloc_reads                                        184     183      -1
xmalloc_getline                                       46       -     -46
fbsplash_main                                       1472    1030    -442
------------------------------------------------------------------------------
(add/remove: 2/1 grow/shrink: 0/2 up/down: 459/-489)          Total: -30 bytes
   text    data     bss     dec     hex filename
 801181     641    7380  809202   c58f2 busybox_old
 801151     641    7380  809172   c58d4 busybox_unstripped