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	2 Jul 2018 14:35:45 -0000
@@ -430,6 +430,14 @@ conv_escape(char *str, char *conv_ch)
 	char ch;
 	char num_buf[4], *num_end;
 
+	if (*str == '\0') {
+		warnx("incomplete escape sequence");
+		rval = 1;
+		value = '\\';
+		*conv_ch = value;
+		return str;
+	}
+
 	ch = *str++;
 
 	switch (ch) {