[PATCH] imm32: Don't generate context unless the window handle is valid.

Mon Apr 16 07:50:37 CDT 2018

This is completely inspired by comment 14 in the mentioned bug [1], but
I'm unsure if this is a correct use of 'IMM_IsCrossThreadAccess()' at
all, and as such, any comments on that will be appreciated!

I imagine it is very possible that this works simply due to the 'else
if' not being true due to this change, resulting into 'NULL' just as
well as forcing it in some other way.

Thanks!

1. https://bugs.winehq.org/show_bug.cgi?id=28861#c14