Update m5310a.c

if (!str) 
    {
        return -1;
    }

devices/m5310a/m5310a.c

@@ -262,6 +262,10 @@ static int m5310a_signal_quality_check(void)
     }
 
     str = strstr(echo.buffer, "+CSQ:");
+    if (!str) 
+    {
+        return -1;
+    }
     sscanf(str, "+CSQ:%d,%d", &rssi, &ber);
     if (rssi == 99) {
         return -1;