nv50/ir: check for origin insn in findOriginForTestWithZero

Function arguments do not have an "origin" instruction, causing a
NULL-pointer dereference without this check.

Signed-off-by: Pierre Moreau <pierre.morrow@free.fr>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/gallium/drivers/nouveau/codegen/nv50_ir_peephole.cpp

@@ -410,6 +410,8 @@ ConstantFolding::findOriginForTestWithZero(Value *value)
    if (!value)
       return NULL;
    Instruction *insn = value->getInsn();
+   if (!insn)
+      return NULL;
 
    if (insn->asCmp() && insn->op != OP_SLCT)
       return insn->asCmp();