Don't crash on failing DRIVER_FindFromHDrvr

[Eric Pouech]

> Why should expecting a NULL return be wrong? It  happens, as the the crashes
> show. 
no. the cases are different. The ones you refer to (mainly in driver.c) 
use a hDrvr passed by the called. So, we have to catch erroneous hDrvr 
values.
The case in lolvldrv.c is different. The hDrvr used is the one returned 
by the call to OpenDriver in the same function. That's why it shouldn't 
fail (unless someone closes the driver between the call to OpenDriver 
and the one to DRIVER_FindFromHDrvr - and yes, winmm functions should be 
made reentrant, why they are not today)
A+