Modify zeroize internal buffers in md modules

Modify all the following functions to zeroize an internal buffer before
exiting the function. The buffer could potentially contain confidential
data read from a file.

* md2_file()
* md4_file()
* md5_file()
* ripemd160_file()
* sha1_file()
* sha256_file()
* sha512_file()
7 files changed