avformat/wavdec: Check if there are 16 bytes before testing them

Fixes: use-of-uninitialized-value
Fixes: 70839/clusterfuzz-testcase-minimized-ffmpeg_dem_W64_fuzzer-5212907590189056

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

libavformat/wavdec.c

@@ -874,8 +874,7 @@ static int w64_read_header(AVFormatContext *s)
     uint8_t guid[16];
     int ret;
 
-    avio_read(pb, guid, 16);
-    if (memcmp(guid, ff_w64_guid_riff, 16))
+    if (avio_read(pb, guid, 16) != 16 || memcmp(guid, ff_w64_guid_riff, 16))
         return AVERROR_INVALIDDATA;
 
     /* riff + wave + fmt + sizes */