bcrypt_pbkdf.c: fix clang10 false positive warning (#563)
bcrypt_pbkdf.c: fix clang10 false positive warning (#563)

File: bcrypt_pbkdf.c

Notes:
blf_enc() takes a number of 64-bit blocks to encrypt, but using
sizeof(uint64_t) in the calculation triggers a warning with
clang 10 because the actual data type is uint32_t. Pass
BCRYPT_BLOCKS / 2 for the number of blocks like libc bcrypt(3)
does.

Ref: https://github.com/openbsd/src/commit/04a2240bd8f465bcae6b595d912af3e2965856de

Fixes #562

Credit:
Viktor Szakats
libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2