Taking substring of readStringUntil without handling timeout, or checking result.

[ed7coyne]

In FirebaseArduino::readEvent() the following code is a bug:
String type = client->readStringUntil('\n').substring(7);

readStringUntil will read until a '\n' OR until it times out, default is one second. This makes substring(7) dangerous. The result of getting a substring out of bounds of the string is undefined (according to: https://www.arduino.cc/en/Tutorial/StringSubstring, which says "Caution: make sure your index values are within the String's length or you'll get unpredictable results.")

At the least we should check the length of the returned string before getting the substring.

[ed7coyne]

btw this will happen if the user calls stream(..) then readEvent() without checking available().