Fix possible panic when using ComposeDecodeHookFunc() with no funcs

[eh-steve]

On < v1.3.3, ComposeDecodeHookFunc() was safe to use with no args, but since decea91 (v1.4.1) it can cause a panic due to it accidentally replacing the input with an untyped nil after the nil check on L405

mapstructure/mapstructure.go

Line 405 in 4664f9e

if input == nil {

This PR restores the previous behaviour with minimal impact to allow easier upgrades

[mitchellh]

Thanks this looks pretty good, I think it'd be easier to initialize data to f from the beginning it looks like.