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.

Beginne damit, den bereitgestellten Arduino-Sketch zu reproduzieren und die an sGlobal = sLocal beteiligten String-Kopier- und Zuweisungspfade zu untersuchen. Vergleiche das Verhalten mit dem in issue #97 vorgeschlagenen Fix und konzentriere dich dabei auf den Fall eines eingebetteten NUL. Erledigt ist es, wenn das Kopieren den vollständigen 12-Byte-Wert einschließlich der Bytes nach dem NUL bewahrt, ohne nicht initialisierte Daten.

Vom Indexierungsmodell aus dem Issue-Text verfasst.