utf8ncpy incorrectly loops when it does not hit null-terminator

[tonymarklove]

Following up on my issue from #50, utf8ncpy doesn't (now) correctly stop at n bytes unless it hits the null-terminator in src. The following code demonstrates the problem:

#include "utf8.h"

int main(int argc, char* argv[]) {
  char buffer[10];
  utf8ncpy(buffer, "foo", 2);
}

Running this program results in a segmentation fault for me, due to n looping round past 0.

Changing the 2 to 3 works, because the null-terminator is hit at the end of the string "foo".