bugfix: check if both begin and end are numbers in UnmarshalJSON

[brunoerg]

In the current code, if we provide a non-number value (e.g. "method":"deriveaddresses","params":["A",["A","B"]]), it might cause a panic. This PR fixes it.

[guggero]

Thanks, LGTM 🎉

[coveralls]

Pull Request Test Coverage Report for Build 10514904620

Details

• 5 of 7 (71.43%) changed or added relevant lines in 1 file are covered.
• 6 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.008%) to 57.235%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
btcjson/walletsvrcmds.go	5	7	71.43%

Files with Coverage Reduction	New Missed Lines	%
mempool/mempool.go	1	66.67%
peer/peer.go	5	74.16%

Totals
Change from base Build 10411760109:	0.008%
Covered Lines:	29845
Relevant Lines:	52145

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💛 - Coveralls

[Roasbeef]

LGTM 📈