Fixed the String::lastIndexOf bug

[a1ext]

The String::lastIndexOf incorrectly finds character \0 behind the string size, so extra check was added.

Description of Change

This pull request fixes the bug in int String::lastIndexOf(char ch, unsigned int fromIndex) const function. The bug itself
is that if the character we are searching is \0 this function returns an index behind bounds of the string itself.
For example the following code prints that it found it at 4th position (points after the end of the actual string):

String test = "test";
int p = test.lastIndexOf('\0');
USBSerial.printf("lastIndexOf: %d\n", p);

output:

lastIndexOf: 4

but it must be -1 because there is no \0 character inside the string itself.

Tests scenarios

I've tested the code on M5Stack-CoreS3 hardware, Arduino-esp32 core v 3.20011.230801 (framework-arduinoespressif32)

Test case when it is fixed:

String test = "test";
int p = test.lastIndexOf('\0');
assert(p == -1);

[CLAassistant]

All committers have signed the CLA.

[github-actions]

	Messages
📖	You might consider squashing your 3 commits (simplifying branch history).

👋 Hello a1ext, we appreciate your contribution to this project!

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[P-R-O-C-H-Y]

LGTM

[VojtechBartoska]

Related docs: https://www.arduino.cc/reference/en/language/variables/data-types/string/functions/lastindexof/