printk: fix string termination for record_print_text()

Commit f0e386e ("printk: fix buffer overflow potential for
print_text()") added string termination in record_print_text().
However it used the wrong base pointer for adding the terminator.
This led to a 0-byte being written somewhere beyond the buffer.

Use the correct base pointer when adding the terminator.

Fixes: f0e386e ("printk: fix buffer overflow potential for print_text()")
Reported-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210124202728.4718-1-john.ogness@linutronix.de

kernel/printk/printk.c

@@ -1399,7 +1399,7 @@ static size_t record_print_text(struct printk_record *r, bool syslog,
 	 * not counted in the return value.
 	 */
 	if (buf_size > 0)
-		text[len] = 0;
+		r->text_buf[len] = 0;
 
 	return len;
 }