Check for identity element is missing during verification

[Robloit]

According to NIST and BSI, during signature verification, the calculated point after shamirs trick should be checked. If it is the identity element or a multiple of it, further evaluation should be aborted. This check is missing and should be included here: https://github.com/kmackay/micro-ecc/blob/b335ee812bfcca4cd3fb0e2a436aab39553a555a/uECC.c#L1588

For reference see these links: NIST: https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-5.pdf (Section 6.4.2 Step 6) BSI: https://www.bsi.bund.de/SharedDocs/Downloads/EN/BSI/Publications/TechGuidelines/TR03111/BSI-TR-03111_V-2-1_pdf.pdf?__blob=publicationFile&v=1 (Section 4.2.1.2 Step 4)