fix type checking of stream element-type

[avodonosov]

Fixes the following use case:

(ironclad:digest-stream :md5 (flexi-streams:make-in-memory-input-stream #(1 2 3)))

Unsupported stream element-type FLEXI-STREAMS:OCTET for stream #<FLEXI-STREAMS::VECTOR-INPUT-STREAM #x302119FBE6AD>.
   [Condition of type SIMPLE-ERROR]

Restarts:
 0: [RETRY] Retry SLIME interactive evaluation request.
 1: [*ABORT] Return to SLIME's top level.
 2: [ABORT-BREAK] Reset this thread
 3: [ABORT] Kill this thread

Backtrace:
  0: (IRONCLAD::UPDATE-DIGEST-FROM-STREAM #S(IRONCLAD:MD5 :AMOUNT 0 :BUFFER #(0 0 0 0 0 0 ...) :BUFFER-INDEX ...) #<FLEXI-STREAMS::VECTOR-INPUT-STREAM #x302119FBE6AD> :BUFFER NIL :START 0 :END NIL)
  1: (#<STANDARD-METHOD IRONCLAD:DIGEST-STREAM (T T)> #S(IRONCLAD:MD5 :AMOUNT 0 :BUFFER #(0 0 0 0 0 0 ...) :BUFFER-INDEX ...) #<FLEXI-STREAMS::VECTOR-INPUT-STREAM #x302119FBE6AD> :BUFFER NIL :START 0 :END ..