ffm: reject invalid codec_id and codec_type

A negative codec_id cannot be handled by the found_decoder API of
AVStream->info: if the codec_id is not recognized, found_decoder is set
to -codec_id, which has to be '<0' according to the API documentation.

This can cause NULL pointer dereferencing in try_decode_frame.

Also make sure the codec_type matches the expected one for codec_id.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavformat/ffmdec.c

@@ -276,6 +276,7 @@ static int ffm2_read_header(AVFormatContext *s)
     AVStream *st;
     AVIOContext *pb = s->pb;
     AVCodecContext *codec;
+    const AVCodecDescriptor *codec_desc;
     int ret;
     int f_main = 0, f_cprv = -1, f_stvi = -1, f_stau = -1;
     AVCodec *enc;
@@ -330,7 +331,20 @@ static int ffm2_read_header(AVFormatContext *s)
             codec = st->codec;
             /* generic info */
             codec->codec_id = avio_rb32(pb);
+            codec_desc = avcodec_descriptor_get(codec->codec_id);
+            if (!codec_desc) {
+                av_log(s, AV_LOG_ERROR, "Invalid codec id: %d\n", codec->codec_id);
+                codec->codec_id = AV_CODEC_ID_NONE;
+                goto fail;
+            }
             codec->codec_type = avio_r8(pb);
+            if (codec->codec_type != codec_desc->type) {
+                av_log(s, AV_LOG_ERROR, "Codec type mismatch: expected %d, found %d\n",
+                       codec_desc->type, codec->codec_type);
+                codec->codec_id = AV_CODEC_ID_NONE;
+                codec->codec_type = AVMEDIA_TYPE_UNKNOWN;
+                goto fail;
+            }
             codec->bit_rate = avio_rb32(pb);
             codec->flags = avio_rb32(pb);
             codec->flags2 = avio_rb32(pb);
@@ -479,6 +493,7 @@ static int ffm_read_header(AVFormatContext *s)
     AVStream *st;
     AVIOContext *pb = s->pb;
     AVCodecContext *codec;
+    const AVCodecDescriptor *codec_desc;
     int i, nb_streams;
     uint32_t tag;
 
@@ -516,7 +531,20 @@ static int ffm_read_header(AVFormatContext *s)
         codec = st->codec;
         /* generic info */
         codec->codec_id = avio_rb32(pb);
+        codec_desc = avcodec_descriptor_get(codec->codec_id);
+        if (!codec_desc) {
+            av_log(s, AV_LOG_ERROR, "Invalid codec id: %d\n", codec->codec_id);
+            codec->codec_id = AV_CODEC_ID_NONE;
+            goto fail;
+        }
         codec->codec_type = avio_r8(pb); /* codec_type */
+        if (codec->codec_type != codec_desc->type) {
+            av_log(s, AV_LOG_ERROR, "Codec type mismatch: expected %d, found %d\n",
+                   codec_desc->type, codec->codec_type);
+            codec->codec_id = AV_CODEC_ID_NONE;
+            codec->codec_type = AVMEDIA_TYPE_UNKNOWN;
+            goto fail;
+        }
         codec->bit_rate = avio_rb32(pb);
         codec->flags = avio_rb32(pb);
         codec->flags2 = avio_rb32(pb);