Unordered chunks could result in trak->end_chunk smaller than trak->start_chunk
in ngx_http_mp4_crop_stsc_data().  Later in ngx_http_mp4_update_stco_atom()
this caused buffer overread while trying to calculate trak->end_offset.

While cropping an stsc atom in ngx_http_mp4_crop_stsc_data(), a 32-bit integer
overflow could happen, which could result in incorrect seeking and a very large
value stored in "samples".  This resulted in a large invalid value of
trak->end_chunk_samples.  This value is further used to calculate the value of
trak->end_chunk_samples_size in ngx_http_mp4_update_stsz_atom().  While doing
this, a large invalid value of trak->end_chunk_samples could result in reading
memory before stsz atom start.  This could potentially result in a segfault.

In some rare cases, graceful shutdown may happen while initializing an HTTP/2
connection.  Previously, such a connection ignored the shutdown and remained
active.  Now it is gracefully closed prior to processing any streams to
eliminate the shutdown delay.

Previously, st->value was passed with NULL data pointer to header handlers.

While inserting a new entry into the dynamic table, first the entry is added,
and then older entries are evicted until table size is within capacity.  After
the first step, the number of entries may temporarily exceed the maximum
calculated from capacity by one entry, which previously caused table overflow.

The easiest way to trigger the issue is to keep adding entries with empty names
and values until first eviction.

The issue was introduced by 987bee4363d1.

Previously a decoder stream was created on demand for sending Section
Acknowledgement, Stream Cancellation and Insert Count Increment.  If conditions
for sending any of these instructions never happen, a decoder stream is not
created at all.  These conditions include client not using the dynamic table and
no streams abandoned by server (RFC 9204, Section 2.2.2.2).  However RFC 9204,
Section 4.2 defines only one condition for not creating a decoder stream:

   An endpoint MAY avoid creating a decoder stream if its decoder sets
   the maximum capacity of the dynamic table to zero.

The change enables pre-creation of the decoder stream at HTTP/3 session
initialization if maximum dynamic table capacity is not zero.  Note that this
value is currently hardcoded to 4096 bytes and is not configurable, so the
stream is now always created.

Also, the change fixes a potential stack overflow when creating a decoder
stream in ngx_http_v3_send_cancel_stream() while draining a request stream by
ngx_drain_connections().  Creating a decoder stream involves calling
ngx_get_connection(), which calls ngx_drain_connections(), which will drain the
same request stream again.  If client's MAX_STREAMS for uni stream is high
enough, these recursive calls will continue until we run out of stack.
Otherwise, decoder stream creation will fail at some point and the request
stream connection will be drained.  This may result in use-after-free, since
this connection could still be referenced up the stack.

Previously chain links could sometimes be dropped instead of being reused,
which could result in increased memory consumption during long requests.

A similar chain link issue in ngx_http_gzip_filter_module was fixed in
da46bfc484ef (1.11.10).

Based on a patch by Sangmin Lee.

Previously, a request body larger than declared in Content-Length resulted in
a 413 status code, because Content-Length was mistakenly used as the maximum
allowed request body, similar to client_max_body_size.  Following the HTTP/3
specification, such requests are now rejected with the 400 error as malformed.

Previously, the response text wasn't initialized and the rewrite module
was sending response body set to NULL.

Found with UndefinedBehaviorSanitizer (pointer-overflow).

Signed-off-by: Piotr Sikora <piotr@aviatrix.com>