demod_flex_next.c: parse_alphanumeric: Fix format string bug

[v-rzh]

The message contents were trusted as a format argument, resulting in a potential format string bug. This commit fixes the issue.

[v-rzh]

@EliasOenal this isn't really a bug like it was in demod_flex.c since add_ch sanitizes % from the pager messages, however if there is any code changes/moves at some point this bug could slip through the cracks, so I think it's best to follow this practice.

[EliasOenal]

Thanks!

[v-rzh]

Thanks!

Thanks for maintaining this project :)