[Backport 3.6] Refactored a minor check in ecp check privkey

[minosgalanakis]

Description

This is a backport of #9494

PR checklist

Please remove the segment/s on either side of the | symbol as appropriate, and add any relevant link/s to the end of the line. If the provided content is part of the present PR remove the # symbol.

• [ ] changelog not required because: is a minor comment
• [ ] development PR provided #9494
• [ ] framework PR provided not required
• [ ] 3.6 PR provided
• [ ] 2.28 PR provided #9496
• tests provided | not required because:

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