Here follow the output of my analysis:
__stdio_fwrite expects at least 1 bytes. fputs_unlocked(S,F) calls
fwrite_unlocked and this calls __stdio_fwrite only if bytes to be
written are > 0, otherwise simply returs 0 (that is correct).
During the parsing of format spec it could happen that __stdio_fwrite
is called passing an empty string and with assertion enabled it will abort.