btcec: validate R and S signature components in RecoverCompact

[onyb]

Issue	GH-1651

Prevent panic in btcec.RecoverCompact by validating signature components. We check if r and s values are:

• Less than the elliptic curve order (N).
• Non-zero.

The RecoverCompact algorithm relies on a multiplicative inverse of the r value. The panic in #1651 was due to the fact that the r value (i.e., x-coordinate of random point R) was 0. Such a point does not lie on the secp256k1 curve either, making the signature invalid.

[coveralls]

Pull Request Test Coverage Report for Build 535078472

• 15 of 15 (100.0%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.02%) to 53.427%

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Files with Coverage Reduction	New Missed Lines	%
database/ffldb/blockio.go	4	92.62%

Totals
Change from base Build 530986489:	0.02%
Covered Lines:	20868
Relevant Lines:	39059

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💛 - Coveralls

[jcvernaleo]

Looks great!
OK