MFV r285292:

Merge upstream fix to eliminate build-breaking gcc warnings of no
importance.

commit: cab33b7a0acba7d2268a23c4383be6167106e549

Update ND_TTEST2 to fix issue 443

Add IS_NOT_NEGATIVE macro.
Avoid these warnings:
- comparison of unsigned expression >= 0 is always true [-Wtype-limits],
- comparison is always true due to limited range of data type [-Wtype-limits].

Reviewed by: adrian
Approved by: jmallett (mentor)
MFC after: 1 month

contrib/tcpdump/netdissect.h

@@ -270,8 +270,11 @@ struct netdissect_options {
  *
  *	http://www.kb.cert.org/vuls/id/162289
  */
+
+#define IS_NOT_NEGATIVE(x) (((x) > 0) || ((x) == 0))
+
 #define ND_TTEST2(var, l) \
-  ((l) >= 0 && \
+  (IS_NOT_NEGATIVE(l) && \
 	((uintptr_t)ndo->ndo_snapend - (l) <= (uintptr_t)ndo->ndo_snapend && \
          (uintptr_t)&(var) <= (uintptr_t)ndo->ndo_snapend - (l)))