avformat/flvdec: Check double before cast in parse_keyframes_index()

Fixes: -2.21166e+304 is outside the range of representable values of type 'long'
Fixes: 29169/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5725452796821504

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

@@ -453,9 +453,13 @@ static int parse_keyframes_index(AVFormatContext *s, AVIOContext *ioc, int64_t m
         }
 
         for (i = 0; i < arraylen && avio_tell(ioc) < max_pos - 1; i++) {
+            double d;
             if (avio_r8(ioc) != AMF_DATA_TYPE_NUMBER)
                 goto invalid;
-            current_array[0][i] = av_int2double(avio_rb64(ioc));
+            d = av_int2double(avio_rb64(ioc));
+            if (isnan(d) || d < INT64_MIN || d > INT64_MAX)
+                goto invalid;
+            current_array[0][i] = d;
         }
         if (times && filepositions) {
             // All done, exiting at a position allowing amf_parse_object