txscript: validate claimscript size

[coveralls]

Pull Request Test Coverage Report for Build 2959954759

• 5 of 19 (26.32%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 51.681%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
txscript/claimscript.go	5	19	26.32%
<!--	Total:	5	19	26.32%	-->

Files with Coverage Reduction	New Missed Lines	%
peer/peer.go	2	76.31%
<!--	Total:	2		-->

Totals
Change from base Build 2858449374:	0.02%
Covered Lines:	22485
Relevant Lines:	43507

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💛 - Coveralls

[moodyjon]

For what it's worth, I was looking at this for ~30min and comparing with lbrycrd. I did not see anything wrong, and no complaints.