Check that scalars are scalars during conversion

[maelle]

(i.e. do not try to index into a zero-length vector with REAL(vec)[0]) — waiting on #1068

[szhorvat]

This was done in tandem with bounds checking for scalar. Same comments apply as in #1140 (comment)

Auto-generated part was done in #1051, manually written code still needs checks using R_check_int_scalar(), R_check_real_scalar(), R_check_bool_scalar()

[maelle]

Ok, then I'll check the box in #840 and close this. Thank you!

[szhorvat]

It wouldn't be a bad idea to keep at least one issue open for including checks in all manually written functions. Of course we want to auto-generate as much as possible, but I don't see this getting to 100%.