Fixes crash noticed in the cbs_userdata patchset.

====ERROR: AddressSanitizer: heap-buffer-overflow on address 0x609000026c89 at pc 0x00010725d37b bp 0x7ffeea04e750 sp 0x7ffeea04e748
READ of size 4 at 0x609000026c89 thread T0
    #0 0x10725d37a in ff_cbs_read_unsigned get_bits.h:274
    #1 0x1072d2767 in ff_cbs_read_a53_user_data cbs_misc_syntax_template.c:119
    #2 0x1078251a7 in h264_metadata_filter h264_metadata_bsf.c:595
    #3 0x105c1321d in output_packet ffmpeg.c:853

0x609000026c89 is located 1 bytes to the right of 8-byte region [0x609000026c80,0x609000026c88)
allocated by thread T0 here:
    #0 0x10aef08d7 in wrap_posix_memalign (libclang_rt.asan_osx_dynamic.dylib:x86_64h+0x578d7)
    #1 0x10aca95e6 in av_malloc mem.c:87
    #2 0x10ac545fe in av_buffer_allocz buffer.c:72
    #3 0x107263b27 in cbs_h264_read_nal_unit cbs_h264_syntax_template.c:722
    #4 0x10725b688 in cbs_read_fragment_content cbs.c:155
Signed-off-by: NAman Gupta <aman@tmm1.net>

Signed-off-by: NAman Gupta <aman@tmm1.net>

A recent version of the standard changed the max and default to 15, from
16 in older versions.  This updates the default to 15 to match, but the
max stays as 16 so that we don't reject older streams.

The user should only interact directly with the data length, not the
payload size.

Otherwise it will hit an assert in the function
ff_cbs_trace_syntax_element() in cbs.c, line 400.
Signed-off-by: NHaihao Xiang <haihao.xiang@intel.com>

This makes the trace output for arrays significantly nicer.

The type of the result of a shift operation is unaffected by the type of
the right operand, so some existing code overflows with undefined behaviour
when the element length is 32.  Add a helper macro to calculate the maximum
value correctly and then use it everywhere this pattern appears.
Found-by: NAndreas Rheinhardt <andreas.rheinhardt@googlemail.com>

It's very large, and is only used in some FMO streams.

If there is exactly one possible SPS but it is not yet active then just
assume that it should be the active one.

(cherry picked from commit acf06f45)
(cherry picked from commit 768eb918)
(cherry picked from commit e7f64191)