VBox-4.2.6: guests fail with assert when dvd drive passthrough is enabled: Expression: pvBuf || enmTxDir == PDMBLOCKTXDIR_NONE

[Yuri]

Guests fail soon after boot in exactly the same way, see log below when 
Pioneer DVD-RW is shared with "Passthrough" option.
This worked few months ago, but I can't tell which version was vbox it 
when it worked.

9.1-STABLE amd64

Below is the end of the log with assert and few lines before that having 
some error (VERR_NOT_IMPLEMENTED) that may or may not be related.

Yuri


---log---
<skipped>
00:00:03.596652 AIO: Async I/O manager not supported 
(rc=VERR_NOT_SUPPORTED). Falling back to simple manager
<skipped>
00:00:03.755364 Chipset cannot do MSI: VERR_NOT_IMPLEMENTED
<skipped>
00:00:36.127426 NAT: DNS#0: 192.168.5.1
00:00:36.127452 NAT: DHCP offered IP address 10.0.2.15
00:00:36.142824 NAT: DHCP offered IP address 10.0.2.15
00:00:37.129084
00:00:37.129086 !!Assertion Failed!!
00:00:37.129087 Expression: pvBuf || enmTxDir == PDMBLOCKTXDIR_NONE
00:00:37.129088 Location  : 
/usr/ports/emulators/virtualbox-ose/work/VirtualBox-4.2.6/src/VBox/Devices/Storage/DrvHostBase.cpp(1250) 
int DRVHostBaseScsiCmd(DRVHOSTBASE*, const uint8_t*, size_t, 
PDMBLOCKTXDIR, void*, uint32_t*, uint8_t*, size_t, uint32_t)