Update the current RTP parser fuzzer to handle header extensions.

This changes the corpus semantics, but libfuzzer should be smart enough to figure it out, and if not then we can add a seed_corpus to help.

BUG=webrtc:4771
NOTRY=true

Review-Url: https://codereview.webrtc.org/2072473002
Cr-Commit-Position: refs/heads/master@{#13384}

webrtc/test/fuzzers/rtp_header_fuzzer.cc

diff --git a/webrtc/test/fuzzers/rtp_header_fuzzer.cc b/webrtc/test/fuzzers/rtp_header_fuzzer.cc
index fcc7377..914a8fb 100644
--- a/webrtc/test/fuzzers/rtp_header_fuzzer.cc
+++ b/webrtc/test/fuzzers/rtp_header_fuzzer.cc
@@ -10,29 +10,35 @@
 
 #include <bitset>
 
-#include "webrtc/modules/rtp_rtcp/source/rtp_packet_received.h"
 #include "webrtc/modules/rtp_rtcp/include/rtp_rtcp_defines.h"
+#include "webrtc/modules/rtp_rtcp/source/rtp_packet_received.h"
 #include "webrtc/modules/rtp_rtcp/source/rtp_utility.h"
 
 namespace webrtc {
+// We decide which header extensions to register by reading one byte
+// from the beginning of |data| and interpreting it as a bitmask over
+// the RTPExtensionType enum. This assert ensures one byte is enough.
+static_assert(kRtpExtensionNumberOfExtensions <= 8,
+              "Insufficient bits read to configure all header extensions. Add "
+              "an extra byte and update the switches.");
+
 void FuzzOneInput(const uint8_t* data, size_t size) {
   if (size <= 1)
     return;
 
-  // We decide which header extensions to register by reading one byte
-  // from the beginning of |data| and interpreting it as a bitmask
-  // over the RTPExtensionType enum. That byte shouldn't also be part
-  // of the packet, so we adjust |data| and |size| to remove it.
+  // Don't use the configuration byte as part of the packet.
   std::bitset<8> extensionMask(data[0]);
   data++;
   size--;
 
   RtpPacketReceived::ExtensionManager extensions;
-  for (int i = 1; i <= kRtpExtensionNumberOfExtensions; i++) {
-    // Skip i=0 which is kRtpExtensionNone i.e. not an actual extension.
-    if (extensionMask[i]) {
-      // We use i as the ID; it's used in negotiation so not relevant.
-      extensions.Register(static_cast<RTPExtensionType>(i), i);
+  for (int i = 0; i < kRtpExtensionNumberOfExtensions; i++) {
+    RTPExtensionType extension_type = static_cast<RTPExtensionType>(i);
+    if (extensionMask[i] && extension_type != kRtpExtensionNone) {
+      // Extensions are registered with an ID, which you signal to the
+      // peer so they know what to expect. This code only cares about
+      // parsing so the value of the ID isn't relevant; we use i.
+      extensions.Register(extension_type, i);
     }
   }