avformat/mov: Fix reel_name size check

Only read str_size bytes from offset 30 of extradata if the extradata is
indeed at least 30 + str_size bytes long.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
(cherry picked from commit ff3fad6b0e)
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavformat/mov.c

@@ -2335,7 +2335,7 @@ FF_ENABLE_DEPRECATION_WARNINGS
                 uint32_t format = AV_RB32(st->codecpar->extradata + 22);
                 if (format == AV_RB32("name") && (int64_t)size >= (int64_t)len + 18) {
                     uint16_t str_size = AV_RB16(st->codecpar->extradata + 26); /* string length */
-                    if (str_size > 0 && size >= (int)str_size + 26) {
+                    if (str_size > 0 && size >= (int)str_size + 30) {
                         char *reel_name = av_malloc(str_size + 1);
                         if (!reel_name)
                             return AVERROR(ENOMEM);