avcodec/cbs_av1_syntax_template: Check num_y_points

"It is a requirement of bitstream conformance that num_y_points is less than or equal to 14."

Fixes: index 24 out of bounds for type 'uint8_t [24]'
Fixes: 19282/clusterfuzz-testcase-minimized-ffmpeg_BSF_AV1_FRAME_MERGE_fuzzer-5747424845103104

Note, also needs a23dd336

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

libavcodec/cbs_av1.h

@@ -256,8 +256,8 @@ typedef struct AV1RawFrameHeader {
     uint8_t  update_grain;
     uint8_t  film_grain_params_ref_idx;
     uint8_t  num_y_points;
-    uint8_t  point_y_value[16];
-    uint8_t  point_y_scaling[16];
+    uint8_t  point_y_value[14];
+    uint8_t  point_y_scaling[14];
     uint8_t  chroma_scaling_from_luma;
     uint8_t  num_cb_points;
     uint8_t  point_cb_value[16];

libavcodec/cbs_av1_syntax_template.c

@@ -1155,7 +1155,7 @@ static int FUNC(film_grain_params)(CodedBitstreamContext *ctx, RWContext *rw,
         return 0;
     }
 
-    fb(4, num_y_points);
+    fc(4, num_y_points, 0, 14);
     for (i = 0; i < current->num_y_points; i++) {
         fbs(8, point_y_value[i],   1, i);
         fbs(8, point_y_scaling[i], 1, i);