win32: Properly handle TlsGetValue returning NULL

win32: Properly handle TlsGetValue returning NULL

virThreadSelf tries to access the virThreadPtr stored in TLS for the
current thread via TlsGetValue. When virThreadSelf is called on a thread
that was not created via virThreadCreate (e.g. the main thread) then
TlsGetValue returns NULL as TlsAlloc initializes TLS slots to NULL.

virThreadSelf can be called on the main thread via this call chain from
virsh

vshDeinit
virEventAddTimeout
virEventPollAddTimeout
virEventPollInterruptLocked
virThreadIsSelf

triggering a segfault as virThreadSelf unconditionally dereferences the
return value of TlsGetValue.

Fix this by making virThreadSelf check the TLS slot value for NULL and
setting the given virThreadPtr accordingly.

Reported by Marcel Müller.