ustream-openssl: fix BIO_method memory leak

ustream-openssl: fix BIO_method memory leak

Fixes following issues as reported by clang-12 LeakSanitizer:

 $ uclient-fetch-san -q -O /dev/null 'https://expired.badssl.com/'
  Direct leak of 96 byte(s) in 1 object(s) allocated from:
      #0 0x49716d in malloc (uclient-fetch-san+0x49716d)
      #1 0x7f551cbabe58 in CRYPTO_zalloc (/usr/lib/x86_64-linux-gnu/libcrypto.so.1.1+0x17ae58)

  Indirect leak of 8 byte(s) in 1 object(s) allocated from:
      #0 0x49716d in malloc (uclient-fetch-san+0x49716d)
      #1 0x7f551cbb51c5 in CRYPTO_strdup (/usr/lib/x86_64-linux-gnu/libcrypto.so.1.1+0x1841c5)

  SUMMARY: AddressSanitizer: 104 byte(s) leaked in 2 allocation(s).

and Valgrind:

  $ valgrind --quiet --leak-check=full uclient-fetch -q -O /dev/null 'https://expired.badssl.com/'
  ==1966== 104 (96 direct, 8 indirect) bytes in 1 blocks are definitely lost in loss record 4 of 9
  ==1966==    at 0x4C31B0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
  ==1966==    by 0x5FC4E58: CRYPTO_zalloc (in /usr/lib/x86_64-linux-gnu/libcrypto.so.1.1)
  ==1966==    by 0x5EF712F: BIO_meth_new (in /usr/lib/x86_64-linux-gnu/libcrypto.so.1.1)
  ==1966==    by 0x5C48039: ustream_bio_new (ustream-io-openssl.c:125)
  ==1966==    by 0x5C48039: ustream_set_io (ustream-io-openssl.c:141)
  ==1966==    by 0x5C47CB0: _ustream_ssl_init (ustream-ssl.c:210)
  ==1966==    by 0x4E4117A: uclient_setup_https (uclient-http.c:914)
  ==1966==    by 0x4E4117A: uclient_http_connect (uclient-http.c:936)
  ==1966==    by 0x401FD9: init_request (uclient-fetch.c:333)
  ==1966==    by 0x401E08: main (uclient-fetch.c:745)

Suggested-by: Pan Chen <serial115200@outlook.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>