fftools/ffmpeg_filter: fix NULL pointer dereference

In close_output(), a dummy frame is created with format NONE passed
to enc_open(), which isn't prepared for it. The NULL pointer
dereference happened at
av_pix_fmt_desc_get(enc_ctx->pix_fmt)->comp[0].depth.

When fgt.graph is NULL, skip fg_output_frame() since there is
nothing to output.

frame #0: 0x0000005555bc34a4 ffmpeg_g`enc_open(opaque=0xb400007efe2db690, frame=0xb400007efe2d9f70) at ffmpeg_enc.c:235:44
frame #1: 0x0000005555bef250 ffmpeg_g`enc_open(sch=0xb400007dde2d4090, enc=0xb400007e4e2daad0, frame=0xb400007efe2d9f70) at ffmpeg_sched.c:1462:11
frame #2: 0x0000005555bee094 ffmpeg_g`send_to_enc(sch=0xb400007dde2d4090, enc=0xb400007e4e2daad0, frame=0xb400007efe2d9f70) at ffmpeg_sched.c:1571:19
frame #3: 0x0000005555bee01c ffmpeg_g`sch_filter_send(sch=0xb400007dde2d4090, fg_idx=0, out_idx=0, frame=0xb400007efe2d9f70) at ffmpeg_sched.c:2154:12
frame #4: 0x0000005555bcf124 ffmpeg_g`close_output(ofp=0xb400007e4e2d85b0, fgt=0x0000007d1790eb08) at ffmpeg_filter.c:2225:15
frame #5: 0x0000005555bcb000 ffmpeg_g`fg_output_frame(ofp=0xb400007e4e2d85b0, fgt=0x0000007d1790eb08, frame=0x0000000000000000) at ffmpeg_filter.c:2317:16
frame #6: 0x0000005555bc7e48 ffmpeg_g`filter_thread(arg=0xb400007eae2ce7a0) at ffmpeg_filter.c:2836:15
frame #7: 0x0000005555bee568 ffmpeg_g`task_wrapper(arg=0xb400007d8e2db478) at ffmpeg_sched.c:2200:21

Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>

fftools/ffmpeg_filter.c

@@ -2835,7 +2835,7 @@ read_frames:
     for (unsigned i = 0; i < fg->nb_outputs; i++) {
         OutputFilterPriv *ofp = ofp_from_ofilter(fg->outputs[i]);
 
-        if (fgt.eof_out[i])
+        if (fgt.eof_out[i] || !fgt.graph)
             continue;
 
         ret = fg_output_frame(ofp, &fgt, NULL);