avcodec/libx265: Don't copy A53 closed captions by default

The SEI handling of libx265 is buggy and can easily lead
to memory corruption: It reuses certain buffers, but when
reusing them it presumes that it is enough for these buffers
to exist and does not check whether they are actually large
enough to hold what is intended to be stored in them.*

Our users are exposed to this because forwarding A53 CC data
is enabled by default. Change this to make it disabled
by default.

"Fixes" tickets #10411, #11052 and (presumably) #10906.

*: See https://trac.ffmpeg.org/ticket/9666#comment:1Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/libx265.c

@@ -948,7 +948,7 @@ static const AVOption options[] = {
     { "tune",        "set the x265 tune parameter",                                                 OFFSET(tune),      AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE },
     { "profile",     "set the x265 profile",                                                        OFFSET(profile),   AV_OPT_TYPE_STRING, { 0 }, 0, 0, VE },
     { "udu_sei",     "Use user data unregistered SEI if available",                                 OFFSET(udu_sei),   AV_OPT_TYPE_BOOL,   { .i64 = 0 }, 0, 1, VE },
-    { "a53cc",       "Use A53 Closed Captions (if available)",                                      OFFSET(a53_cc),    AV_OPT_TYPE_BOOL,   { .i64 = 1 }, 0, 1, VE },
+    { "a53cc",       "Use A53 Closed Captions (if available)",                                      OFFSET(a53_cc),    AV_OPT_TYPE_BOOL,   { .i64 = 0 }, 0, 1, VE },
     { "x265-params", "set the x265 configuration using a :-separated list of key=value parameters", OFFSET(x265_opts), AV_OPT_TYPE_DICT,   { 0 }, 0, 0, VE },
 #if X265_BUILD >= 167
     { "dolbyvision", "Enable Dolby Vision RPU coding", OFFSET(dovi.enable), AV_OPT_TYPE_BOOL, {.i64 = FF_DOVI_AUTOMATIC }, -1, 1, VE, .unit = "dovi" },