Fix a possible segfault in wcsxfrm(3) and wcsxfrm_l(3).
If the length of the source wide character string, passed in via the
"size_t n" parameter, is set to zero, the function should only return
the required length for the destination wide character string. In this
case, it should *not* attempt to write to the destination, so the "dst"
parameter is permitted to be NULL.
However, when the internally called _collate_wxfrm() function returns an
error, such as when using the "C" locale, as a fallback wcscpy(3) or
wcsncpy(3) are used. But if the input length is zero, wcsncpy(3) will
be called with a length of -1! If the "dst" parameter is NULL, this
will immediately result in a segfault, or if "dst" is a valid pointer,
it will most likely result in unexpectedly overwritten memory.
Fix this by explicitly checking for an input length greater than zero,
before calling wcsncpy(3).
Note that a similar situation does not occur in strxfrm(3), the plain
character version of this function, as it uses strlcpy(3) for the error
case. The strlcpy(3) function does not write to the destination if the
input length is zero.