nimble/transport: Skip H4 event parsing if received event is too big

If received HCI event is too big to handle just return error and
stop parsing instead of overwriting data in memeory.

Author: sjanc

nimble/transport/common/hci_h4/src/hci_h4.c

@@ -168,6 +168,9 @@ hci_h4_sm_w4_header(struct hci_h4_sm *h4sm, struct hci_h4_input_buffer *ib)
         }
 
         h4sm->exp_len = h4sm->hdr[1] + 2;
+        if (h4sm->exp_len > MYNEWT_VAL(BLE_TRANSPORT_EVT_SIZE)) {
+            return -1;
+        }
         break;
     case HCI_H4_ISO:
         assert(h4sm->allocs && h4sm->allocs->iso);