Clean up EVP_DecryptFinal_ex()

Clean up EVP_DecryptFinal_ex()

Rework the code to use the usual variable names, return early if we
have block size 1 and unindent the remainder of the code for block
sizes 8 and 16. Rework the padding check to be less acrobatic and
copy the remainder of the plain text into out using memcpy() rather
than a for loop.

input/ok jsing