Why couldn't he survive in the center of the sun? Not saying he WOULD, but it's never been tested, and is his shield works as described, then it would automatically repel flames without exception.
That said, the gravitational fields in the sun might be outside of Magneto's ability to control, and may wreak havoc on his shields, which exposes him to the sun's heat and kills him.
Eh, who knows.
Either way, Magneto encloses Torch in a gravitational field that simulates a vacuum and prevents air from getting in. Extinguishes Torch. Knocks him out. Heats up a pastrami sandwhich with Torch's smoldering body on the way out.