gallivm: Prevent double delete on LLVM 3.6

std::unique_ptr takes ownership of MM, and a double delete could ensure
in case of an error,  as pointed out by Chris Vine in
https://bugs.freedesktop.org/show_bug.cgi?id=89387

Reviewed-by: Chris Vine <chris@cvine.freeserve.co.uk>
1 file changed