Fix #272: check `c_obj_create` error code for `OBJ_R_OID_EXISTS`. #339

Job	Run time
cygwin (windows-2019, win64, -DOQS_ALGS_ENABLED=STD)	3m 18s
msvc (windows-2019, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A-masm)	6h 0m 4s
msvc_native (windows-2019, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	3m 11s
msvc_native (windows-2022, win64, -DOQS_ALGS_ENABLED=STD, no-shared no-fips VC-WIN64A, .CMake/too...	3m 6s
	6h 9m 39s

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