Check for overflow and NaN when doing double -> igraph_integer_t conversion.

[maelle]

From #840.

[szhorvat]

This is done for the auto-generated part, see #1051 and R_check_int_scalar().

It still needs to be done for the manually written functions, but that does not involve Stimulus. A few manually written functions were done in #1069. PR #1068 (not yet merged) makes a further improvement.

[krlmlr]

From #1071.