avformat/flvdec: Check data before casting

Fixes: -nan is outside the range of representable values of type 'long'
Fixes: signed integer overflow: 1000 * -9223372036854775808 cannot be represented in type 'long'
Fixes: 34890/clusterfuzz-testcase-minimized-ffmpeg_dem_FLV_fuzzer-5334208657620992

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/flvdec.c

@@ -682,7 +682,11 @@ static int amf_parse_object(AVFormatContext *s, AVStream *astream,
             av_dict_set(&s->metadata, key, str_val, 0);
         } else if (amf_type == AMF_DATA_TYPE_STRING) {
             av_dict_set(&s->metadata, key, str_val, 0);
-        } else if (amf_type == AMF_DATA_TYPE_DATE) {
+        } else if (   amf_type == AMF_DATA_TYPE_DATE
+                   && isfinite(date.milliseconds)
+                   && date.milliseconds > INT64_MIN/1000
+                   && date.milliseconds < INT64_MAX/1000
+                  ) {
             // timezone is ignored, since there is no easy way to offset the UTC
             // timestamp into the specified timezone
             avpriv_dict_set_timestamp(&s->metadata, key, 1000 * (int64_t)date.milliseconds);