fix: integer vectors are validated before transferring them to the C library

[szhorvat]

Effects of this change:

• Fixes cluster_label_prop() crashes R when using fixed labels #1434 and eliminates a potential way to crash igraph
• Potential small performance penalty. Probably inconsequential, but not measured.
• This can potentially trigger an UBSan warning, and that's okay. The warning will only be triggered when passing invalid values. If this happens, the test suite or examples should be changed.
• Values that were in the past blindly rounded are now checked. May cause revdepcheck failures.

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