Fix flex/stax bug for signing Michelin messages #32

Job	Run time
build_docker_tezos_ocaml	0s
Build application (nanos)	1m 42s
Build application (nanosp)	1m 45s
Build application (nanox)	1m 46s
Build application (stax)	1m 56s
Build application (flex)	1m 54s
generate_samples_unit_tests	0s
integration_tests_basic (nanos)	4m 13s
integration_tests_samples	0s
integration_tests_basic (nanosp)	4m 7s
integration_tests_basic (nanox)	4m 5s
integration_tests_basic (stax)	2m 45s
integration_tests_basic (flex)	2m 59s
	27m 12s

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