types.ChainAnteDecorators() flaws

[alessio]

While I've been writing test cases for the package types, I stumble upon a couple of small issues with sdk.ChainAnteDecorators():

1. When the chain argument is an empty slice, the function panics. The godoc comment does not mention such a case.
2. The second if clause never evaluates true:

   cosmos-sdk/types/handler.go

   Line 33 in 3349c97

   if len(chain) == 1 {

   
   When len(chain) == 1 a Terminator had already been appended in the previous if block.

Wrt 1, there are two solutions:

• let it panic and mention this case in the documentation
• just fix return nil when len(chain) == 0 and add a godoc line. This would be an ABI breakage (i.e. a change in function's semantic), though I think it's not a big deal.

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