Fix possible buffer overrun and/or unportable behavior in pg_md5_encrypt()

Fix possible buffer overrun and/or unportable behavior in pg_md5_encrypt()
if salt_len == 0.  This seems to be mostly academic, since nearly all calling
code paths guarantee nonempty salt; the only case that doesn't is
PQencryptPassword where the caller could mistakenly pass an empty username.
So, fix it but don't bother backpatching.  Per ljb.