[xml] Access Violation in xmlMutexUnlock

["Oliver Stöneberg" <oliverst online de>]

A few times I got an access violation in xmlMutexUnlock, when 
ReleaseMutex tried to release the tok->mutex, which was 0x0000000 in 
the case it tried to free it. Unfortunately I *can't* reproduce this, 
but it happened quite often to me by now :(

I am using libxml2 2.5.10 in a DLL build with Visual Studio 6.0. The 
libxml2 is linked static, uses iconv and native threads.
The portion of code I am using the libxml2 in, is just reading a 
string from memory, then goes to a specific node and atrribute, reads 
the value and closes it again. Everything is closed and free'd 
properly, because it's quite a small amount of code. It happened, 
when 3 threads accssed the DLL. All the other code *is* thread-safe 
(tested and verfied by some test-programs)

Did anyone else ever had this problem? Could it be a problem in the 
threads? 
I am heavily trying to get a reproducable situation, but it's not 
looking good so far.