avformat/flacdec: Return correct error-codes on read-failure

Forward errors from `avio_read` directly. When `avio_read` sees EOF before
expected bytes can be read, consistently return `AVERROR_INVALIDDATA`

We used to return `AVERROR(AVERROR_INVALIDDATA)` when failing to read
metadata block headers. `AVERROR_INVALIDDATA` is already negative, so
wrapping in `AVERROR` leads to double-negation.

We used to return `AVERROR(EIO)` when failing to read extended metadata.
However, many times, the IO-layer is not at fault, the input data is simply
corrupted (truncated), so we return `AVERROR_INVALIDDATA` here as well.

---

Tomas: changed to use AVERROR_EOF

libavformat/flacdec.c

@@ -80,8 +80,13 @@ static int flac_read_header(AVFormatContext *s)
 
     /* process metadata blocks */
     while (!avio_feof(s->pb) && !metadata_last) {
-        if (avio_read(s->pb, header, 4) != 4)
-            return AVERROR_INVALIDDATA;
+        ret = avio_read(s->pb, header, 4);
+        if (ret < 0) {
+            return ret;
+        } else if (ret != 4) {
+            return AVERROR_EOF;
+        }
+
         flac_parse_block_header(header, &metadata_last, &metadata_type,
                                    &metadata_size);
         switch (metadata_type) {
@@ -95,8 +100,11 @@ static int flac_read_header(AVFormatContext *s)
             if (!buffer) {
                 return AVERROR(ENOMEM);
             }
-            if (avio_read(s->pb, buffer, metadata_size) != metadata_size) {
-                RETURN_ERROR(AVERROR(EIO));
+            ret = avio_read(s->pb, buffer, metadata_size);
+            if (ret < 0) {
+                RETURN_ERROR(ret);
+            } else if (ret != metadata_size) {
+                RETURN_ERROR(AVERROR_EOF);
             }
             break;
         /* skip metadata block for unsupported types */