tty: n_gsm: add sanity check for gsm->receive in gsm_receive_buf()

A null pointer dereference can happen when attempting to access the
"gsm->receive()" function in gsmld_receive_buf(). Currently, the code
assumes that gsm->recieve is only called after MUX activation.
Since the gsmld_receive_buf() function can be accessed without the need to
initialize the MUX, the gsm->receive() function will not be set and a
NULL pointer dereference will occur.

Fix this by avoiding the call to "gsm->receive()" in case the function is
not initialized by adding a sanity check.

Call Trace:
 <TASK>
 gsmld_receive_buf+0x1c2/0x2f0 drivers/tty/n_gsm.c:2861
 tiocsti drivers/tty/tty_io.c:2293 [inline]
 tty_ioctl+0xa75/0x15d0 drivers/tty/tty_io.c:2692
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:870 [inline]
 __se_sys_ioctl fs/ioctl.c:856 [inline]
 __x64_sys_ioctl+0x193/0x200 fs/ioctl.c:856
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

Link: https://syzkaller.appspot.com/bug?id=bdf035c61447f8c6e0e6920315d577cb5cc35ac5
Fixes: 01aecd91

 ("tty: n_gsm: fix tty registration before control channel open")
Cc: stable <stable@kernel.org>
Reported-and-tested-by:  <syzbot+e3563f0c94e188366dbb@syzkaller.appspotmail.com>
Signed-off-by: Mazin Al Haddad <mazinalhaddad05@gmail.com>
Link: https://lore.kernel.org/r/20220814015211.84180-1-mazinalhaddad05@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>