Log Message:
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use BUF_MEM_free instead of OPENSSL_free to free BUF_MEM in str_copy
While reviewing a PR, I noticed that str_copy in conf_def.c allocates a
BUF_MEM with BUF_MEM_free, but frees it with OPENSSL_free. While this
is fine here since we take ownership of the buf mem data pointer, it was
a bit hard to parse.
Change it to follow the usual pattern of orphaning the BUF_MEM data by
setting buf->data to NULL prior to using the standard BUF_MEM_free call