Fix strsep_quote() on strings without quotes.

For strings without quotes and escapes dstptr and srcptr are equal, so
zeroing *dstptr before checking *srcptr is not a good idea.  In practice
it means that in -maproot=65534:65533 everything after the colon is lost.

The problem was there since r293305, but before r346976 it was covered by
improper strsep_quote() usage.

PR:		238725
MFC after:	3 days
Sponsored by:	iXsystems, Inc.

usr.sbin/mountd/mountd.c

@@ -385,8 +385,8 @@ strsep_quote(char **stringp, const char *delim)
 		*dstptr++ = *srcptr++;
 	}
 
-	*dstptr = 0; /* Terminate the string */
 	*stringp = (*srcptr == '\0') ? NULL : srcptr + 1;
+	*dstptr = 0; /* Terminate the string */
 	return (retval);
 }