Fix OOB write in show_status_fmt

This fixes a buffer overflow which can be triggered by reducing the terminal size to about 25 columns (using default settings) while minicom is running.

snprintf returns the number of characters (excluding the terminating null byte) which would have been written to the final string if enough space had been available. Thus, if a string gets truncated, the buffer index must not be advanced by the return value of snprintf, as this may cause an out-of-bounds access in subsequent writes. Instead, the buffer index should be advanced by MIN(size, snprintf(..., size)), which returns the actual number of characters written.

This error is caught by the bufi < COLS condition present in the for loop, except for the case where two writes occur in a single iteration, as is the case for format character b, if the socket type is neither Unix nor TCP.