TTY: ntty, add one more sanity check
TTY: ntty, add one more sanity check

With the previous patch, we fixed another bug where read_buf was freed
while we still was in n_tty_read. We currently check whether read_buf
is NULL at the start of the function. Add one more check after we wake
up from waiting for input.

Signed-off-by: Jiri Slaby <dfbf39eb5bf303a3db5454d815cb8c8888cbadf9@suse.cz>
Cc: Alan Cox <91e38e63b890fbb214c8914809fde03c73e7f24d@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <4645f7897fd33786a2ee1264d590b3c400559d85@suse.de>

TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs