ash: fix incorrect path in describe_command

ash: fix incorrect path in describe_command

$ PATH=/extra/path:/usr/sbin:/usr/bin:/sbin:/bin \
> busybox sh -xc 'command -V ls; command -V ls; command -Vp ls; command -vp ls'
+ command -V ls
ls is /bin/ls
+ command -V ls
ls is a tracked alias for /bin/ls
+ command -Vp ls
ls is a tracked alias for (null)
+ command -vp ls
Segmentation fault

describe_command should respect `path' argument. Looking up in the hash table
may gives incorrect index in entry.u.index and finally causes incorrect output
or SIGSEGV.

function                                             old     new   delta
describe_command                                     386     313     -73

Signed-off-by: Youfu Zhang <zhangyoufu@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>