openssl: fix uninitialized memory access

ASN1_STRING_to_UTF8() passes an ASN1_STRING to ASN1_STRING_set() but
forgot to initialize the `length` field.

Fixes the following valgrind error:

  $ valgrind -q --track-origins=yes --num-callers=19 \
      out/Debug/node test/simple/test-tls-client-abort.js
  ==2690== Conditional jump or move depends on uninitialised value(s)
  ==2690==    at 0x784B69: ASN1_STRING_set (asn1_lib.c:382)
  ==2690==    by 0x809564: ASN1_mbstring_ncopy (a_mbstr.c:204)
  ==2690==    by 0x8090F0: ASN1_mbstring_copy (a_mbstr.c:86)
  ==2690==    by 0x782F1F: ASN1_STRING_to_UTF8 (a_strex.c:570)
  ==2690==    by 0x78F090: asn1_string_canon (x_name.c:409)
  ==2690==    by 0x78EF17: x509_name_canon (x_name.c:354)
  ==2690==    by 0x78EA7D: x509_name_ex_d2i (x_name.c:210)
  ==2690==    by 0x788058: ASN1_item_ex_d2i (tasn_dec.c:239)
  ==2690==    by 0x7890D4: asn1_template_noexp_d2i (tasn_dec.c:746)
  ==2690==    by 0x788CB6: asn1_template_ex_d2i (tasn_dec.c:607)
  ==2690==    by 0x78877A: ASN1_item_ex_d2i (tasn_dec.c:448)
  ==2690==    by 0x7890D4: asn1_template_noexp_d2i (tasn_dec.c:746)
  ==2690==    by 0x788CB6: asn1_template_ex_d2i (tasn_dec.c:607)
  ==2690==    by 0x78877A: ASN1_item_ex_d2i (tasn_dec.c:448)
  ==2690==    by 0x787C93: ASN1_item_d2i (tasn_dec.c:136)
  ==2690==    by 0x78F5E4: d2i_X509 (x_x509.c:141)
  ==2690==    by 0x7C9B91: PEM_ASN1_read_bio (pem_oth.c:81)
  ==2690==    by 0x7CA506: PEM_read_bio_X509 (pem_x509.c:67)
  ==2690==    by 0x703C9A: node::crypto::SecureContext::AddRootCerts(v8::Arguments const&) (node_crypto.cc:497)
  ==2690==  Uninitialised value was created by a stack allocation
  ==2690==    at 0x782E89: ASN1_STRING_to_UTF8 (a_strex.c:560)