MSVC: Replace variable-length automatic arrays with alloca

MSVC is not C99 compatible[1]. Errors:

  test_ciphers.c(312,25): error C2057: expected constant expression
  test_ciphers.c(312,25): error C2466: cannot allocate an array of constant size 0
  test_ciphers.c(312,26): error C2133: 'c': unknown size

Link: https://docs.microsoft.com/en-us/cpp/c-language/ansi-conformance


Signed-off-by: Vitaly Chikunov <vt@altlinux.org>