too late null checks

[Ted Unangst]

Bugs found with Coverity's automated analysis.  In each case, either the 
NULL check is unnecessary, or it's too late because the variable was 
already deref'd.

dev/firewire/fwohci.c:dump_db (debugging function)
pp can not be null (crash if it is)

dev/mpt/mpt_pci.c:mpt_detach
a null mpt won't survive mpt_prt

netinet/tcp_subr.c:tcp_mtudisc
tp has already been dereferenced to obtain isipv6 result

usb/dev/urio.c:urio_attach
if uaa->device can be null, usbd_devinfo already used it.

cam/cam_xpt.c:xptplistperiphfunc
check at line 2532 looks wrong.  perhaps should be *pdrv == NULL?

dev/twe/twe.c:twe_ioctl
if you make it to line 552, tr is definitely not null.

kern/vfs_subr.c:reassignbufs (4.10)
1408 while (tbp && tbp->b_lblkno < 0)
1409 tdp = TAILQ_PREV(tbp, buflists, b_vnbufs);
1410 TAILQ_INSERT_AFTER(listheadp, tbp, bp, b_vnbufs);
if the first while() test fails, tailq insert will crash.
i'm not sure where this code went in current.  some other bugs of a 
similar nature have already been fixed or the code removed in current, 
let me know if you are still interested in 4.10 results.