Do not accept invalid octal character values (>= 0400).

This also avoid possible buffer overflows (e.g. previously `tr -s '\400'' or
similars lead to them).

Index: str.c
===================================================================
RCS file: /cvsroot/src/usr.bin/tr/str.c,v
retrieving revision 1.29
diff -u -p -r1.29 str.c
--- str.c	11 Aug 2013 01:54:35 -0000	1.29
+++ str.c	25 May 2018 20:03:42 -0000
@@ -417,6 +417,8 @@ backslash(STR *s)
 	}
 	if (cnt) {
 		/* We saw digits, so return their value */
+		if (val >= OOBCH)
+			errx(1, "Invalid octal character value");
 		return val;
 	}
 	if (ch == '\0') {