Fix TAINTED_SCALAR false positives of Coverity

Coverity Analysis 2019.03 incorrectly marks the input argument
of base64_encode(), and conseuqnetly base64_encode_alloc(), as
tainted_data_sink because it sees byte-level operations on the input.
This one-line annotation makes Coverity suppress the following false
positives:

Error: TAINTED_SCALAR:
lib/luks2/luks2_digest_pbkdf2.c:117: tainted_data_argument: Calling function "crypt_random_get" taints argument "salt".
lib/luks2/luks2_digest_pbkdf2.c:157: tainted_data: Passing tainted variable "salt" to a tainted sink.

Error: TAINTED_SCALAR:
lib/luks2/luks2_keyslot_luks2.c:445: tainted_data_argument: Calling function "crypt_random_get" taints argument "salt".
lib/luks2/luks2_keyslot_luks2.c:448: tainted_data: Passing tainted variable "salt" to a tainted sink.