multimedia/svt-av1: unbreak ffmpeg < 5.1 encoding after 7d7c819ad05e

$ ffmpeg -hide_banner -i foo.y4m -c:v libsvtav1 -y foo.mp4
[...]
Segmentation fault
(lldb) bt

• thread #1, name = 'ffmpeg', stop reason = signal SIGSEGV: invalid address (fault address: 0x438)
  • frame #0: 0x00000008372f34b9 libSvtAv1Enc.so.1`svt_aom_copy_metadata_buffer(dst=0x000000089e044680, src=0x0000000000000438) at EbMetadataHandle.c:108:26 frame #1: 0x00000008372e67c4 libSvtAv1Enc.so.1`copy_input_buffer(scs=0x0000000868186380, dst=0x000000089e044680, dst_y8b=0x000000089e047380, src=0x0000000820287b20, pass=0) at EbEncHandle.c:4820:13 frame #2: 0x00000008372e64d4 libSvtAv1Enc.so.1`svt_av1_enc_send_picture(svt_enc_component=0x000000085f30de60, p_buffer=0x0000000820287b20) at EbEncHandle.c:4865:9 frame #3: 0x000000082449de42 libavcodec.so.58`eb_receive_packet at libsvtav1.c:433:9 frame #4: 0x000000082449dc7a libavcodec.so.58`eb_receive_packet(avctx=0x000000085f309300, pkt=0x000000085f312c00) at libsvtav1.c:503 frame #5: 0x000000082419f960 libavcodec.so.58`encode_receive_packet_internal(avctx=0x000000085f309300, avpkt=0x000000085f312c00) at encode.c:301:15 frame #6: 0x000000082419f893 libavcodec.so.58`avcodec_send_frame(avctx=0x000000085f309300, frame=0x0000000000000000) at encode.c:387:15 frame #7: 0x0000000000231e9a ffmpeg`transcode at ffmpeg.c:1995:23 frame #8: 0x0000000000231ce8 ffmpeg`transcode at ffmpeg.c:4825 frame #9: 0x000000000022da03 ffmpeg`main(argc=<unavailable>, argv=<unavailable>) at ffmpeg.c:5010:9 frame #10: 0x0000000000215ad0 ffmpeg`_start(ap=<unavailable>, cleanup=<unavailable>) at crt1_c.c:75:7

Regressed by: https://gitlab.com/AOMediaCodec/SVT-AV1/-/commit/921c1877c050
See also: https://git.ffmpeg.org/gitweb/ffmpeg.git/commitdiff/4243da4ff42e