[9fans] snprintf buffer overrun

[Porlock]

Plan 9's implementation of the standard C functions snprintf and
vsnprintf have a buffer overrun bug.

If the buffer length equals the output length (without the terminating
null), then one too many characters is written to the buffer.

For example,
              snprintf(buf, 4, "ABCD");

will write 5 characters to buf.

Attached is a short program to illustrate this, which gives the
following output :-

% 8c printftest.c && 8l printftest.8
% ./8.out
  A   B   C   D   \0  *   *   *   *   *   *   *   *   *   *   *



#include <u.h>
#include <libc.h>
#include <stdio.h>

void main()
{
    char buf[16];
    int i;
    memset(buf, '*', sizeof(buf));
    snprintf(buf, 4, "ABCD");
    for (i = 0; i < sizeof(buf); ++i) {
        if (buf[i])
            print("  %c ", buf[i]);
        else
            print("  \\0");
    }
    print("\n");
}