avcodec/pnm_parser: clear state if buffer is discarded

Fixes: Assertion failure
Fixes: 14484/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PGMYUV_fuzzer-5150016408125440

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

libavcodec/pnm_parser.c

@@ -69,6 +69,7 @@ retry:
         if (pnmctx.bytestream < pnmctx.bytestream_end) {
             if (pc->index) {
                 pc->index = 0;
+                pnmpc->ascii_scan = 0;
             } else {
                 unsigned step = FFMAX(1, pnmctx.bytestream - pnmctx.bytestream_start);