avcodec/bintext: Check font height

Fixes: division by zero
Fixes: 15257/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_BINTEXT_fuzzer-5757352881422336

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit bfb58bdd70)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/bintext.c

@@ -63,6 +63,10 @@ static av_cold int decode_init(AVCodecContext *avctx)
             av_log(avctx, AV_LOG_ERROR, "not enough extradata\n");
             return AVERROR_INVALIDDATA;
         }
+        if (!s->font_height) {
+            av_log(avctx, AV_LOG_ERROR, "invalid font height\n");
+            return AVERROR_INVALIDDATA;
+        }
     } else {
         s->font_height = 8;
         s->flags = 0;