Update ND_BYTES_AVAILABLE_AFTER() macro for better accuracy

With this change the number of bytes available in the captured data
given by the macro is 0 when the argument is greater than or equal
to ndo_snapend or less than ndo_packetp (e.g. pointer underflow).

i.e.:
If p is like p3, the macro gives 0.
If p is like p1, the macro gives 0.

|------------------|-----------------------|-----------------|
      p1  <   ndo_packetp  <=  p2  <  ndo_snapend   <=   p3

Update the ascii_print(), hex_and_ascii_print_with_offset() and
hex_print_with_offset() functions accordingly.

This is a follow-up to 07a7f33.

Author: fxlb

netdissect.h

@@ -391,7 +391,7 @@ nd_trunc_longjmp(netdissect_options *ndo)
  * Number of bytes remaining in the captured data, starting at the
  * byte pointed to by the argument.
  */
-#define ND_BYTES_AVAILABLE_AFTER(p) ND_BYTES_BETWEEN((p), ndo->ndo_snapend)
+#define ND_BYTES_AVAILABLE_AFTER(p) ((const u_char *)(p) < ndo->ndo_packetp ? 0 : ND_BYTES_BETWEEN((p), ndo->ndo_snapend))
 
 /*
  * Check (expression_1 operator expression_2) for invalid packet with

print-ascii.c

@@ -67,7 +67,7 @@ ascii_print(netdissect_options *ndo,
 	int truncated = FALSE;
 
 	ndo->ndo_protocol = "ascii";
-	caplength = (ndo->ndo_snapend > cp) ? ND_BYTES_AVAILABLE_AFTER(cp) : 0;
+	caplength = ND_BYTES_AVAILABLE_AFTER(cp);
 	if (length > caplength) {
 		length = caplength;
 		truncated = TRUE;
@@ -113,7 +113,7 @@ hex_and_ascii_print_with_offset(netdissect_options *ndo, const char *indent,
 	char hexstuff[HEXDUMP_SHORTS_PER_LINE*HEXDUMP_HEXSTUFF_PER_SHORT+1], *hsp;
 	char asciistuff[ASCII_LINELENGTH+1], *asp;
 
-	caplength = (ndo->ndo_snapend > cp) ? ND_BYTES_AVAILABLE_AFTER(cp) : 0;
+	caplength = ND_BYTES_AVAILABLE_AFTER(cp);
 	if (length > caplength) {
 		length = caplength;
 		truncated = TRUE;
@@ -181,7 +181,7 @@ hex_print_with_offset(netdissect_options *ndo,
 	u_int nshorts;
 	int truncated = FALSE;
 
-	caplength = (ndo->ndo_snapend > cp) ? ND_BYTES_AVAILABLE_AFTER(cp) : 0;
+	caplength = ND_BYTES_AVAILABLE_AFTER(cp);
 	if (length > caplength) {
 		length = caplength;
 		truncated = TRUE;