avcodec/bintext: sanity check dimensions

Fixes: Timeout
Fixes: 6277/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_XBIN_fuzzer-6047202288861184

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

libavcodec/bintext.c

@@ -35,6 +35,8 @@
 #include "bintext.h"
 #include "internal.h"
 
+#define FONT_WIDTH 8
+
 typedef struct XbinContext {
     AVFrame *frame;
     int palette[16];
@@ -91,6 +93,9 @@ static av_cold int decode_init(AVCodecContext *avctx)
             break;
         }
     }
+    if (avctx->width < FONT_WIDTH || avctx->height < s->font_height)
+        return AVERROR_INVALIDDATA;
+
 
     s->frame = av_frame_alloc();
     if (!s->frame)
@@ -113,8 +118,6 @@ av_unused static void hscroll(AVCodecContext *avctx)
     }
 }
 
-#define FONT_WIDTH 8
-
 /**
  * Draw character to screen
  */