fix OOB on String.UTF8.encode using nullTerminated = true

[romdotdog]

fixes #2021

Examples

before it worked like this

String.UTF8.byteLength("a\0", true); // -> 2
String.UTF8.encode("a\0", true); // writes 0x61, 0x00, and then another 0x00

now it works like this

String.UTF8.byteLength("a\0", true); // -> 2
String.UTF8.encode("a\0", true); // writes 0x61, 0x00 and breaks because of the null

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[dcodeIO]

Thanks :)