sm2_sig_verify(): Do not call BN_CTX_end() without BN_CTX_start() #1074

Job	Run time
run-checker (enable-asan enable-ubsan no-shared no-asm -DOPENSSL_SMALL_FOOTPRINT)	10m 10s
run-checker (no-ct)	5m 26s
run-checker (no-dso)	5m 17s
run-checker (no-dynamic-engine)	5m 27s
run-checker (no-ec2m enable-fips)	5m 49s
run-checker (no-engine no-shared)	4m 39s
run-checker (no-err)	5m 25s
run-checker (no-filenames)	5m 18s
run-checker (enable-ubsan no-asm -DOPENSSL_SMALL_FOOTPRINT -fno-sanitize=alignment)	7m 56s
run-checker (no-module)	5m 16s
run-checker (no-ocsp)	5m 22s
run-checker (no-pinshared)	5m 36s
run-checker (no-srp)	5m 24s
run-checker (no-srtp)	5m 22s
run-checker (no-ts)	5m 39s
run-checker (no-integrity-only-ciphers)	5m 33s
run-checker (enable-weak-ssl-ciphers)	5m 37s
run-checker (enable-zlib)	5m 36s
run-checker (enable-pie)	5m 26s
	1h 50m 18s

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