Repositorio Institucional de la Universidad de Oviedo

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We thank A. Tseytlin for useful remarks. D.R-G is partially supported by the Spanish government grant MINECO-16-FPA2015-63667-P. He also acknowledges support from the Principado de Asturias through the grant FC-GRUPIN-IDI/2018/000174 J.G.R. acknowledges financial support from projects 2017-SGR-929, MINECO grant PID2019-105614GBC21, and from the State Agency for Research of the Spanish Ministry of Science and Innovation through the “Unit of Excellence María de Maeztu 2020-2023” (CEX2019-000918-M).