avcodec/ptx: Check that the input contains at least one line

Fixes: Timeout (19sec -> 44ms)
Fixes: 17816/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PTX_fuzzer-5704459950227456

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

libavcodec/ptx.c

@@ -55,6 +55,9 @@ static int ptx_decode_frame(AVCodecContext *avctx, void *data, int *got_frame,
 
     buf += offset;
 
+    if (buf_end - buf < w * bytes_per_pixel)
+        return AVERROR_INVALIDDATA;
+
     if ((ret = ff_set_dimensions(avctx, w, h)) < 0)
         return ret;