On an empty line input, a C3 with UART console would panic while attempting
to output the new Lua prompt. The backtrace shows a xQueueSemaphoreTake
with uxItemSize==0 as the panic cause, deep inside the uart driver, invoked
via vfs_uart and vfs_console layers, from printf.
Similarly, the printf for outputting a backspace/erase sequence would also
trigger a panic.
This workaround (of not mixing fflush() with printf) is likely merely hiding
a deeper issue, but it appears to be consistent. Plus, printf with no args
and a user-supplied format string is a no-no and should be fixed anyway.
70c7437cdc