This change adjusts the stack pointer during CBC decryption. The code
was previously using the red zone across function calls and valgrind
thinks that the "unused" stack is undefined after a function call.
Initial fork from f2d678e6e89b6508147086610e985d4e8416e867 (1.0.2 beta).
(This change contains substantial changes from the original and
effectively starts a new history.)
