WGL: Assert assigns dwThreadId variable

In https://gitlab.freedesktop.org/mesa/mesa/-/blob/d6287a94b697ffe12a4e576a38943cdf4e90cdb0/src/gallium/frontends/wgl/stw_tls.c#L301, an assert is assigning a variable instead of checking for equality. I don't know what this code does (and if the assignment is actually intentional), but it certainly looks like a bug.

assert(data->dwThreadId = GetCurrentThreadId());

I grepped for and found this assignment after coming across a similar situation in an old revision of some Intel driver code.