btcec: check if recovered pk is at point of infinity

[MariusVanDerWijden]

This adds a check to ensure that the recovered pk is not at the point of infinity

[coveralls]

Pull Request Test Coverage Report for Build 1230974468

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• 9 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.02%) to 52.903%

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Files with Coverage Reduction	New Missed Lines	%
mempool/mempool.go	1	66.75%
database/ffldb/blockio.go	4	92.62%
peer/peer.go	4	75.95%

Totals
Change from base Build 1193116021:	0.02%
Covered Lines:	21021
Relevant Lines:	39735

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💛 - Coveralls

[jcvernaleo]

OK