Uninitalised memory when copying String with embedded NUL character

When you create a String object with an embedded NUL character, and you copy this String, the memory after the NUL byte is not copied, leading to uninitialised memory being used.

Here's sample code to show the error:
```
String sGlobal;

void dumpString(const String &s)
{
Serial.print("Got a string of length ");
Serial.println(s.length());
Serial.print(">");
for (size_t t = 0; t < s.length(); ++t) {
if (s.charAt(t) != '\0' && isascii(s.charAt(t))) {
Serial.print(s.charAt(t));
} else if (s.charAt(t) == '\0') {
Serial.print("\\0");
} else {
Serial.print("\\x");
Serial.print(s.charAt(t), HEX);
}
}
Serial.print("<");

Serial.println();
}

void encode(String &s)
{
while (s.length() < 12)
{
s += ' ';
}

Serial.println("s in encode is, after filling with spaces:");
dumpString(s);

s.setCharAt(11, '!');
s.setCharAt(10, 'd');
s.setCharAt(9, 'l');
s.setCharAt(8, 'r');
s.setCharAt(7, 'o');
s.setCharAt(6, 'w');
s.setCharAt(5, '\0');
s.setCharAt(4, 'o');
s.setCharAt(3, 'l');
s.setCharAt(2, 'l');
s.setCharAt(1, 'e');
s.setCharAt(0, 'H');

Serial.println("s in encode is, after setting its chars:");
dumpString(s);

}

void test() {
String sLocal;
Serial.println("sLocal in test is, after init:");
dumpString(sLocal);
Serial.println("sGlobal in test is, after init:");
dumpString(sGlobal);

encode(sLocal);
Serial.println("sLocal in test is, after encode:");
dumpString(sLocal);
Serial.println("sGlobal in test is, after encode:");
dumpString(sGlobal);

sGlobal = sLocal;
Serial.println("sGlobal in test is, after assignment:");
dumpString(sGlobal);
}

void setup()
{
Serial.begin(115200);

test();
}

void loop()
{
}
```

Output of the code:
```
Local in test is, after init:
Got a string of length 0
><
sGlobal in test is, after init:
Got a string of length 0
><
s in encode is, after filling with spaces:
Got a string of length 12
> <
s in encode is, after setting its chars:
Got a string of length 12
>Hello\0world!<
sLocal in test is, after encode:
Got a string of length 12
>Hello\0world!<
sGlobal in test is, after encode:
Got a string of length 0
><
sGlobal in test is, after assignment:
Got a string of length 12
>Hello\0n\xFFFFFFEF\xFFFFFFD6\xFFFFFFFF\<
```

Please merge #97 to fix this issue, or at least use ``memcpy()`` instead of ``strcpy()`` to initialise the data.

Comienza reproduciendo el sketch de Arduino proporcionado e inspeccionando las rutas de copia y asignación de String implicadas en sGlobal = sLocal. Compara el comportamiento con el fix propuesto en issue #97, centrándote en el caso de un NUL incrustado. Se considera terminado cuando la copia conserva el valor completo de 12 bytes, incluidos los bytes posteriores al NUL, sin datos no inicializados.

Escrito por el modelo de indexación a partir del texto del issue.