When there is no format to be found after a %, get_fmt_mapping returns
NULL. It then gets called against strlen, which is a typical SEGV.
Check for NULL aginst mapping, which signals a null format being
specified.

there was a request to improve the error message, but the whole function
has non-informative errors. (both functions, actually, since the code is
duplicated)
as such, instead of catching it directly, address the assumption that
led to the SIGSEGV and let it be caught by the functions themselves,
thus reusing existing error messages (which can then be improved
separately).