solver: forward nil return value from sharedOp.Exec

Exec could sometimes return nil, which would then result in a
segmentation fault, as the code attempts to access a field in the nil
result to return. This patch introduces a sanity check before accessing
any parts of the field.

Signed-off-by: Justin Chadwell <me@jedevc.com>

solver/jobs.go

@@ -872,6 +872,9 @@ func (s *sharedOp) Exec(ctx context.Context, inputs []Result) (outputs []Result,
 		return nil, nil, err
 	}
 	r := res.(*execRes)
+	if r == nil {
+		return nil, nil, nil
+	}
 	return unwrapShared(r.execRes), r.execExporters, nil
 }