Use the right constants in comparisons.  This is currently a nop, as
MIN_RXD == MIN_TXD and MAX_RXD == MAX_TXD.

Reviewed by:	Eric Joyner @ Intel
MFC after:	1 week
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 after:	1 month