Fix possible out-of-bounds read for empty escape sequence (i.e. a `\' alone).

The single `\' is treated as `\\' and a warning is printed and the exit status
raised (like other unknown escape sequences).

Index: printf.c
===================================================================
RCS file: /cvsroot/src/usr.bin/printf/printf.c,v
retrieving revision 1.37
diff -u -p -r1.37 printf.c
--- printf.c	16 Jun 2015 22:54:10 -0000	1.37
+++ printf.c	30 Jun 2018 12:27:50 -0000
@@ -469,6 +469,13 @@ conv_escape(char *str, char *conv_ch)
 	case 't':	value = '\t';	break;	/* tab */
 	case 'v':	value = '\v';	break;	/* vertical-tab */
 
+	case '\0':
+		warnx("empty escape sequence");
+		rval = 1;
+		value = '\\';
+		str--;
+		break;
+
 	default:
 		warnx("unknown escape sequence `\\%c'", ch);
 		rval = 1;