nv50/ir: Initialize BindArgumentsPass member sub in constructor.

Fix defect reported by Coverity.

Uninitialized pointer field (UNINIT_CTOR)
uninit_member: Non-static class member sub is not initialized in
this constructor nor in any functions that it calls.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7229>

src/gallium/drivers/nouveau/codegen/nv50_ir_from_tgsi.cpp

@@ -1692,7 +1692,7 @@ private:
 
    class BindArgumentsPass : public Pass {
    public:
-      BindArgumentsPass(Converter &conv) : conv(conv) { }
+      BindArgumentsPass(Converter &conv) : conv(conv), sub(NULL) { }
 
    private:
       Converter &conv;