mailboxAlloc():  Fix another mailbox bug
mailboxAlloc():  Fix another mailbox bug

Missed one in previous commit --- mboxptr->count and mboxptr->start were
not guaranteed to be initialized before mboxptr->state, potentially
creating a race with a concurrent call to mailboxCount().

ZeroTau/xinu,osa1/osdev,rahsinha2/XINU-OS,ZeroTau/xinu,robert-w-gries/xinu,ZeroTau/xinu,rahsinha2/XINU-OS,robert-w-gries/xinu,rahsinha2/XINU-OS,osa1/osdev,robert-w-gries/xinu,osa1/osdev