strcpy.3: Fix example code for strncpy, which could pass an incorrect length

strcpy.3: Fix example code for strncpy, which could pass an incorrect length

The example code currently passes `buflen - 1` to `strncpy`,
however the length parameter to `strncpy` is `size_t`, which is
unsigned.  This means that when `buflen` is zero, the cast of `-1`
to unsigned will result in passing `UINT_MAX` as the length.
Obviously, that would be incorrect and could cause `strncpy` to
write well beyond the buffer passed.

The easy solution is to wrap the whole code in the `buflen > 0`
check, rather then just the part of the code that applies the null
terminator.

Signed-off-by: Matthew Kilgore <mattkilgore12@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>