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

In the Linux kernel, the following vulnerability has been resolved: tty: ngsm: add sanity check for gsm-receive in gsmreceivebuf A null pointer dereference can happen when attempting to access the "gsm-receive" function in gsmldreceivebuf. Currently, the code assumes that gsm-recieve is only call...