udhcpc: fix segmentation fault on empty bin opt

The following caused udhcpc to segfault:
  busybox udhcpc -i lo -s /dev/null -x 0x3d:

function                                             old     new   delta
udhcp_str2optset                                     629     641     +12

Signed-off-by: Michal Kazior <michal@plume.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
1 file changed