Replace REAL()[0] by REAL_0(), and similar

[krlmlr]

Flagged by CRAN:

diff -Nru orig/igraph/src/rinterface_extra.c patches/igraph/src/rinterface_extra.c
--- orig/igraph/src/rinterface_extra.c	2024-10-18 07:23:06.000000000 -0400
+++ patches/igraph/src/rinterface_extra.c	2024-12-03 11:51:27.514912912 -0500
@@ -4584,6 +4584,8 @@
                                  SEXP palgo) {
 
   igraph_t g;
+  if (Rf_length(pfrom) < 1)
+    Rf_error("empty pfrom");
   igraph_integer_t from=(igraph_integer_t) REAL(pfrom)[0];
   igraph_vs_t to;
   igraph_vector_int_t to_data;

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