Merge pull request #8788 from jboeuf/fix_jwt_verifier

src/core/lib/security/credentials/jwt/jwt_verifier.c

src/core/lib/security/credentials/jwt/jwt_verifier.h

src/core/lib/support/string.c

src/core/lib/support/string.h

test/core/security/create_jwt.c

test/core/security/jwt_verifier_test.c

test/core/security/verify_jwt.c

test/core/support/string_test.c