fix(modem): Fix to allow MSC frame (SIM800 CMUX) after v1.0.2

Fixes the regression from 8edbac6 that added more strict conditions
for CMUX protocol, but didn't allow MSC frames that might be sent during
initializaiton by SIM800.

Closes #366

components/esp_modem/src/esp_modem_cmux.cpp

@@ -144,7 +144,7 @@ bool CMux::data_available(uint8_t *data, size_t len)
             return false;
         }
     } else if ((type & FT_UIH) == FT_UIH && dlci == 0) { // notify the internal DISC command
-        if (len > 0 && (data[0] & 0xE1) == 0xE1) {
+        if ((len > 0 && (data[0] & 0xE1) == 0xE1) || (data == nullptr)) {
             // Not a DISC, ignore (MSC frame)
             return true;
         }