Fix #272: check `c_obj_create` error code for `OBJ_R_OID_EXISTS`. #564

Job	Run time
Security checks	6m 24s
aarch64 cross-compilation	5m 49s
linux_baseline	3m 10s
linux_intel (openssl-3.1.0)	5m 27s
linux_baseline (-DOQS_KEM_ENCODERS=ON)	3m 26s
linux_intel (master)	5m 58s
	30m 14s

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