fix: R API — remove outlawed R_UnboundValue

[maelle]

Avoids non-API calls to Rf_findVar(), Rf_findVarInFrame(), and removes all uses of the now-outlawed R_UnboundValue from code compiled on R >= 4.5.0, fixing build failures on R-devel.

• R_UnboundValue as the ifnotfound sentinel in Rx_igraph_get_pointer and Rx_igraph_graph_version is replaced with R_NilValue.
• The dead-code check identity_func == R_UnboundValue after Rf_findFun in Rx_igraph_safe_eval_in_env is removed (Rf_findFun throws on failure and never returns R_UnboundValue).
• The sole remaining R_UnboundValue reference stays inside the #else block (R < 4.5.0), which is only compiled on older R where the symbol is available.

• By submitting this pull request, I assign the copyright of my contribution to The igraph development team.

[schochastics]

@krlmlr can we close this or is this still necessary?

[krlmlr]

I totally missed this, sorry. I quite like the approach, the R_UnboundValue usage still might be a problem.

[krlmlr]

Let's see what the builds on the main branch say.

[krlmlr]

Let's see if R_UnboundValue comes back as an error here.

[krlmlr]

@copilot: Remove use of the now outlawed R_UnboundValue .

[Copilot]

@copilot: Remove use of the now outlawed R_UnboundValue .

Done in 169c104. Replaced all R_UnboundValue usages in code compiled on R >= 4.5.0:

• Rx_igraph_get_pointer: changed ifnotfound and comparisons to use R_NilValue
• Rx_igraph_graph_version: same
• Rx_igraph_safe_eval_in_env: removed dead-code check (since Rf_findFun throws on failure, it never returns R_UnboundValue)

The sole remaining R_UnboundValue is inside the #else block (R < 4.5.0), which is only compiled on older R where the symbol is available.