Don't ovewrite complex member VCMPacket::generic_descriptor when fuzzing.

In https://webrtc-review.googlesource.com/c/src/+/102720 a new complex member
was added to VCMPacket. This member was overwritten with random data in the
fuzzer, which put it in an invalid state. To avoid that we save/restore it.

Bug: chromium:891597
Change-Id: I7b489afa727a028a542fbec55a4ee27ac54fa698
Reviewed-on: https://webrtc-review.googlesource.com/c/103462
Reviewed-by: Alex Loiko <aleloi@webrtc.org>
Commit-Queue: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24972}

test/fuzzers/packet_buffer_fuzzer.cc

@@ -34,11 +34,18 @@ void FuzzOneInput(const uint8_t* data, size_t size) {
     memcpy(&video_header_backup, &packet.video_header,
            sizeof(packet.video_header));
 
+    uint8_t generic_descriptor_backup[sizeof(packet.generic_descriptor)];
+    memcpy(&generic_descriptor_backup, &packet.generic_descriptor,
+           sizeof(packet.generic_descriptor));
+
     helper.CopyTo(&packet);
 
     memcpy(&packet.video_header, &video_header_backup,
            sizeof(packet.video_header));
 
+    memcpy(&packet.generic_descriptor, &generic_descriptor_backup,
+           sizeof(packet.generic_descriptor));
+
     // The packet buffer owns the payload of the packet.
     uint8_t payload_size;
     helper.CopyTo(&payload_size);