Fix too long (seed length >12 chars) challenge handling.

1. " ext" length should be included into OPIE_CHALLENGE_MAX (as all places

of opie code expects that).

2. Overflow check in challenge.c is off by 1 even with corrected

OPIE_CHALLENGE_MAX

3. When fallback to randomchallenge() happens and rval is 0 (i.e.

challenge is too long), its value should be set to error state too.

To demonstrate the bug, run opiepasswd with valid seed:
opiepasswd -s 1234567890123456
and notice that it falls back to randomchallenge() (i.e. no
1234567890123456 in the prompt).

PR: 191511
Submitted by: mitsururike@gmail.com (partially)
MFC after: 1 week