Fix incomplete initialization bug: BIO_new(BIO_f_asn1()) neglected

Fix incomplete initialization bug: BIO_new(BIO_f_asn1()) neglected
initializing five of the fields in BIO_ASN1_BUF_CTX (prefix,
prefix_free, suffix, suffix_free, ex_arg), inviting a segfault in
a subsequent call from the application program to BIO_write(3)
because subroutines of that function assume that the function
pointers are either NULL or valid.

Fix this by using the less error-prone calloc(3) idiom.

While here, inline asn1_bio_init() at the only call site
in asn1_bio_new() to simplify the code and make it easier to read.

Bug found and initial patch by me,
this version (with inlining) by and OK tb@.