avcodec/ccaption_dec: Add a blank like at the end to avoid rollup reading from outside

Fixes: index 20 out of bounds for type 'const char *[4][128]'
Fixes: 14367/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_CCAPTION_fuzzer-5718819672162304
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ccaption_dec.c

@@ -212,10 +212,10 @@ static const unsigned char pac2_attribs[32][3] = // Color, font, ident
 
 struct Screen {
     /* +1 is used to compensate null character of string */
-    uint8_t characters[SCREEN_ROWS][SCREEN_COLUMNS+1];
-    uint8_t charsets[SCREEN_ROWS][SCREEN_COLUMNS+1];
-    uint8_t colors[SCREEN_ROWS][SCREEN_COLUMNS+1];
-    uint8_t fonts[SCREEN_ROWS][SCREEN_COLUMNS+1];
+    uint8_t characters[SCREEN_ROWS+1][SCREEN_COLUMNS+1];
+    uint8_t charsets[SCREEN_ROWS+1][SCREEN_COLUMNS+1];
+    uint8_t colors[SCREEN_ROWS+1][SCREEN_COLUMNS+1];
+    uint8_t fonts[SCREEN_ROWS+1][SCREEN_COLUMNS+1];
     /*
      * Bitmask of used rows; if a bit is not set, the
      * corresponding row is not used.