Add wc_Sha256Free after the final use #258

Job	Run time
Build wolfSSL (v5.8.4-stable)	4s
Build OpenSSH	5s
Build wolfSSL (master)	6s
Build and test wolfSSH (master, --enable-smallstack)	29s
Build and test wolfSSH (v5.8.4-stable, --enable-smallstack)	21s
Build and test wolfSSH (v5.8.4-stable)	23s
Build and test wolfSSH (master)	25s
	1m 53s

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