I just pulled up an on-line tire size/diameter calculator.
The 245/35R21 comes up with a diameter of 27.8 inches
The 245/45R19 comes up with a diameter of 27.7 inches, or a difference shown as about 0.4% smaller.
That difference is about the same as having roughly 1.5-2/32nd of an inch difference in tread depth on the tire on one side versus the other.
So I think you'll be ok if you're just doing this for a few days until you can get a tire repaired/replaced. Realize you may feel a difference in handling due to different tire stiffness and tread width, but seems like the two sizes are pretty well matched unless the on-line calculator I'm using is lying to me.