Windows: Don't attempt to copy a string to a zero length buffer

It won't cause harm since strcpy_s() deals with zero length buffers,
but it invokes the invalid parameter handler, which can disrupt
execution on debug builds.

lib/roken/strlcpy.c

@@ -43,7 +43,8 @@ strlcpy (char *dst, const char *src, size_t dst_cch)
 {
     errno_t e;
 
-    e = strcpy_s(dst, dst_cch, src);
+    if (dst_cch > 0)
+        e = strncpy_s(dst, dst_cch, src, _TRUNCATE);
 
     return strlen (src);
 }