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

In the Linux kernel, the following vulnerability has been resolved:

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