MFC r271287:

Merge from NetBSD:

PR/49185: Conrad Meyer: strvisx: Handle zero-length input strings
gracefully.  (don't abuse 0 to mean compute string length internally)

PR:		193447
Submitted by:	Conrad Meyer <conrad.meyer@isilon.com>
MFC r272706:
Fix a bug introduced in
https://svnweb.freebsd.org/base?view=revision&revision=272347