fix crash in ecp_nistz256_point_add_affine() #20903

Job	Run time
run-checker (no-cmp)	5m 2s
run-checker (no-cms)	5m 12s
run-checker (no-dgram)	4m 49s
run-checker (no-dh)	5m 3s
run-checker (no-dtls)	5m 19s
run-checker (no-ec)	4m 38s
run-checker (no-ecx)	5m 46s
run-checker (no-http)	5m 12s
run-checker (no-legacy)	5m 19s
run-checker (no-sock)	4m 37s
run-checker (enable-ssl-trace)	5m 14s
run-checker (no-stdio)	2m 31s
run-checker (no-threads)	5m 8s
run-checker (no-thread-pool)	5m 11s
run-checker (no-default-thread-pool)	5m 7s
run-checker (no-tls)	4m 59s
run-checker (no-tls1_2)	5m 19s
run-checker (no-tls1_3)	4m 56s
run-checker (enable-trace enable-fips)	6m 37s
run-checker (no-ui)	5m 27s
run-checker (no-quic)	4m 59s
	1h 46m 25s

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