VPP-1576: fix a class of spurious warnings

Change-Id: I52198e97397bb6f3b5c0c30c44aac34c17650d4d
Signed-off-by: Dave Barach <dave@barachs.net>

src/vppinfra/format.c

@@ -412,6 +412,11 @@ va_format (u8 * s, const char *fmt, va_list * va)
   if (f > g)
     vec_add (s, g, f - g);
 
+#ifdef __COVERITY__
+  if (s == 0)
+    return (u8 *) "liar liar pants on fire s can't be zero!";
+#endif
+
   return s;
 }