Missing input checks to GetInput on alt-da

[anupsv]

Bug Description
There is no nil checks for the input comm CommitmentData which causes the panics when being used. Would be good to have checks against the input.

Steps to Reproduce

var commit op_plasma.CommitmentData
_, err := daClient.GetInput(ts.Ctx, commit)

Expected behavior
Should return an error as "MalformedData" or something similar

Environment Information:
Any environment

Configurations:
N/A

Logs:

panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x20 pc=0x1019f2fe8]

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