Fix incorrect data delivery skips after gap in HTTP Content-Range.

The logic for determining whether a gap was entirely within a MIME
entity body was not asking the current entity, which may be better able
to answer that question if it was using the Content-Range header and
thus knows if the gap exceeds the length of the body that's still
expected.

Addresses BIT-1247

src/analyzer/protocol/http/HTTP.cc

@@ -609,10 +609,10 @@ void HTTP_Message::Done(const int interrupted, const char* detail)
 
 int HTTP_Message::Undelivered(int64_t len)
 	{
-	if ( ! top_level )
+	HTTP_Entity* e = current_entity ? current_entity
-		return 0;
+	                                : static_cast<HTTP_Entity*>(top_level);
 
-	if ( ((HTTP_Entity*) top_level)->Undelivered(len) )
+	if ( e && e->Undelivered(len) )
 		{
 		content_gap_length += len;
 		return 1;

testing/btest/scripts/base/protocols/http/content-range-gap-skip.bro

@@ -0,0 +1,26 @@
+# @TEST-EXEC: bro -r $TRACES/http/content-range-gap-skip.trace %INPUT
+
+# In this trace, we should be able to determine that a gap lies
+# entirely within the body of an entity that specifies Content-Range,
+# and so further deliveries after the gap can still be made.
+
+global got_gap = F;
+global got_data_after_gap = F;
+
+event http_entity_data(c: connection, is_orig: bool, length: count,
+                       data: string)
+	{
+	if ( got_gap )
+		got_data_after_gap = T;
+	}
+
+event content_gap(c: connection, is_orig: bool, seq: count, length: count)
+	{
+	got_gap = T;
+	}
+
+event bro_done()
+	{
+	if ( ! got_data_after_gap )
+		exit(1);
+	}