nswitch() segfaults with no value/output pairs #87

Summary

nswitch() aborts the R process when called with no value/output pairs. It should raise a normal R error.

Reproduce

library(kit)
nswitch(1:3)

Result on R 4.6.2 (macOS):

 *** caught segfault ***
address 0x8000002, cause 'invalid permissions'

Traceback:
 1: nswitch(1:3)
An irrecoverable exception occurred. R is aborting now ...

nswitch(1:3, default = 0L) crashes the same way.

Cause

R/call.R:12 passes list(...) to CnswitchR. With no pairs, that list is length 0.

In src/nswitch.c, n = length(args) is 0, so the even-length check passes, and the code immediately reads PTR_ETL(args, 1) (line 42) and PTR_ETL(args, 0) (line 46) without checking that args has at least two elements. PTR_ETL on an empty list is out of bounds.

Expected

A clear R error, for example:

Error: '...' must contain at least one value/output pair.

or the same error vswitch() gives for zero-length values:

Error: Argument 'values' cannot be zero-length vector.

Fix

In nswitchR(), after n = n / 2, add:

if (n == 0) {
  error("Argument '...' must contain at least one value/output pair.");
}

before any PTR_ETL(args, ...) access.

This is pre-existing and unrelated to #86; I found it while reviewing #74.