avformat/ilbc: Check avio_read() for failure

Fixes: use of uninitialized value
Fixes: 42537627/clusterfuzz-testcase-minimized-fuzzer_protocol_memory-6656646223298560-cut

Found-by: ossfuzz
Reported-by: Kacper Michajlow
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit e30d957a9b)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/ilbc.c

@@ -61,7 +61,8 @@ static int ilbc_read_header(AVFormatContext *s)
     AVStream *st;
     uint8_t header[9];
 
-    avio_read(pb, header, 9);
+    if (avio_read(pb, header, 9) != 9)
+        return AVERROR_INVALIDDATA;
 
     st = avformat_new_stream(s, NULL);
     if (!st)