Fast interpreter: issue with decoding block types

It seems that the fast interpreter can miscompute the length of a block type. I think this happens for multi-byte type indexes. I get this trace for the piece of code below:

jerome@catalpa:~/tmp/bugs$ ~/sources/wizard-engine/bin/spectest.x86-64-linux -ti bug.bin.wast 
##+bug.bin.wast
+1:   block[[] -> [i32 i32]]
+3:   unreachable
##-fail: bug.bin.wast @ 21:2 assert_return expected [], got trap[UNREACHABLE]

(module
  (type (func)) (type (func)) (type (func)) (type (func))
  (type (func)) (type (func)) (type (func)) (type (func))
  (type (func)) (type (func)) (type (func)) (type (func))
  (type (func)) (type (func)) (type (func)) (type (func))
  (type (func)) (type (func)) (type (func)) (type (func))
  (type (func)) (type (func)) (type (func)) (type (func))
  (type (func)) (type (func)) (type (func)) (type (func))
  (type (func)) (type (func)) (type (func)) (type (func))
  (type (func)) (type (func)) (type (func)) (type (func))
  (type (func)) (type (func)) (type (func)) (type (func))
  (type (func)) (type (func)) (type (func)) (type (func))
  (type (func)) (type (func)) (type (func)) (type (func))
  (type (func)) (type (func)) (type (func)) (type (func))
  (type (func)) (type (func)) (type (func)) (type (func))
  (type (func)) (type (func)) (type (func)) (type (func))
  (type (func)) (type (func)) (type (func)) (type (func))
  (type (func))
  (type $res (func (result i32 i32)))
  (func (export "main")
     (block (type $res)
        (return))
     (drop)
     (drop)
  )
)
(assert_return (invoke "main"))

(module definition binary
  "\00\61\73\6d\01\00\00\00\01\c9\81\80\80\00\42\60"
  "\00\00\60\00\00\60\00\00\60\00\00\60\00\00\60\00"
  "\00\60\00\00\60\00\00\60\00\00\60\00\00\60\00\00"
  "\60\00\00\60\00\00\60\00\00\60\00\00\60\00\00\60"
  "\00\00\60\00\00\60\00\00\60\00\00\60\00\00\60\00"
  "\00\60\00\00\60\00\00\60\00\00\60\00\00\60\00\00"
  "\60\00\00\60\00\00\60\00\00\60\00\00\60\00\00\60"
  "\00\00\60\00\00\60\00\00\60\00\00\60\00\00\60\00"
  "\00\60\00\00\60\00\00\60\00\00\60\00\00\60\00\00"
  "\60\00\00\60\00\00\60\00\00\60\00\00\60\00\00\60"
  "\00\00\60\00\00\60\00\00\60\00\00\60\00\00\60\00"
  "\00\60\00\00\60\00\00\60\00\00\60\00\00\60\00\00"
  "\60\00\00\60\00\00\60\00\00\60\00\00\60\00\00\60"
  "\00\00\60\00\02\7f\7f\03\82\80\80\80\00\01\00\07"
  "\88\80\80\80\00\01\04\6d\61\69\6e\00\00\0a\8f\80"
  "\80\80\00\01\89\80\80\80\00\00\02\c1\00\0f\0b\1a"
  "\1a\0b"
)
(module instance)
(assert_return (invoke "main"))

Start by running the supplied bug.bin.wast reproduction with bin/spectest.x86-64-linux -ti and compare the fast interpreter's block-type decoding for the multi-byte type index. Trace why the block is reported with the wrong result types; done means the supplied assert_return passes without an UNREACHABLE trap.