- Fix how hexdump parses escape strings

From the NetBSD bug:
The way how hexdump(1) parses escape sequences has some bugs.
It shows up when an escape sequence is used as the non-last character
of a format string.

PR:		bin/144722
Submitted by:	gcooper
Approved by:	rpaulo
Obtained from:	NetBSD
MFC after:	1 week

usr.bin/hexdump/parse.c

@@ -255,7 +255,9 @@ rewrite(FS *fs)
 					sokay = NOTOKAY;
 			}
 
-			p2 = p1 + 1;		/* Set end pointer. */
+			p2 = *p1 ? p1 + 1 : p1;	/* Set end pointer -- make sure
+						 * that it's non-NUL/-NULL first
+						 * though. */
 			cs[0] = *p1;		/* Set conversion string. */
 			cs[1] = '\0';
 
@@ -449,13 +451,21 @@ escape(char *p1)
 	char *p2;
 
 	/* alphabetic escape sequences have to be done in place */
-	for (p2 = p1;; ++p1, ++p2) {
-		if (!*p1) {
-			*p2 = *p1;
-			break;
-		}
-		if (*p1 == '\\')
-			switch(*++p1) {
+	for (p2 = p1; *p1; p1++, p2++) {
+		/* 
+		 * Let's take a peak at the next item and see whether or not
+		 * we need to escape the value...
+		 */
+		if (*p1 == '\\') {
+
+			p1++;
+
+			switch(*p1) {
+			/* A standalone `\' */
+			case '\0':
+				*p2 = '\\';
+				*++p2 = '\0';
+				break;
 			case 'a':
 			     /* *p2 = '\a'; */
 				*p2 = '\007';
@@ -482,7 +492,12 @@ escape(char *p1)
 				*p2 = *p1;
 				break;
 			}
+
+		} else
+			*p2 = *p1;
+
 	}
+
 }
 
 void