By Phil Stewart BRASILIA (Reuters) – U.S. Defense Secretary Lloyd Austin said on Wednesday he had spoken with U.S. House Speaker Nancy Pelosi and given her a security assessment but any comments about a trip she might make to Taiwan would have to come from her office. China has issued stern warnings to U.S. officials about Pelosi’s possible visit to Taiwan, a Chinese foreign ministry spokesman said on Monday. Pelosi has not confirmed a potential trip to Taiwan. Under its one-China policy, the United States does not have official diplomatic relations with Taiwan, but is bound by U.S. law to pro…