GH-40886: [R] Cryptic error when creating Arrow array from POSIXct with invalid time zones (#49714)

### Rationale for this change

Horrible error with invalid timezones

### What changes are included in this PR?

Do more checking of type

### Are these changes tested?

Yup

### Are there any user-facing changes?

No

### AI usage

Done with Codex, but I went through it myself and am happy with it.

* GitHub Issue: #40886

Authored-by: Nic Crane <thisisnic@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>

Author: thisisnic

r/src/type_infer.cpp

@@ -72,6 +72,8 @@ std::shared_ptr<arrow::DataType> InferArrowTypeFromVector<INTSXP>(SEXP x) {
     if (Rf_isNull(tzone_sexp)) {
       auto systzone_sexp = cpp11::package("base")["Sys.timezone"];
       return timestamp(TimeUnit::MICRO, CHAR(STRING_ELT(systzone_sexp(), 0)));
+    } else if (TYPEOF(tzone_sexp) != STRSXP) {
+      cpp11::stop("`tzone` attribute of a `POSIXct` vector must be a character vector");
     } else {
       return timestamp(TimeUnit::MICRO, CHAR(STRING_ELT(tzone_sexp, 0)));
     }
@@ -89,6 +91,8 @@ std::shared_ptr<arrow::DataType> InferArrowTypeFromVector<REALSXP>(SEXP x) {
     if (Rf_isNull(tzone_sexp)) {
       auto systzone_sexp = cpp11::package("base")["Sys.timezone"];
       return timestamp(TimeUnit::MICRO, CHAR(STRING_ELT(systzone_sexp(), 0)));
+    } else if (TYPEOF(tzone_sexp) != STRSXP) {
+      cpp11::stop("`tzone` attribute of a `POSIXct` vector must be a character vector");
     } else {
       return timestamp(TimeUnit::MICRO, CHAR(STRING_ELT(tzone_sexp, 0)));
     }

r/tests/testthat/test-type.R

@@ -53,6 +53,25 @@ test_that("infer_type() infers from R type", {
   )
 })
 
+test_that("infer_type() errors clearly for POSIXct with invalid tzone", {
+  x <- as.POSIXct("2019-02-14 13:55:05", tz = "UTC")
+  attr(x, "tzone") <- 123
+
+  expect_error(
+    infer_type(x),
+    "`tzone` attribute of a `POSIXct` vector must be a character vector"
+  )
+
+  # Also check zero-length POSIXct
+  x <- as.POSIXct(x = NULL)
+  attr(x, "tzone") <- 123
+
+  expect_error(
+    infer_type(x),
+    "`tzone` attribute of a `POSIXct` vector must be a character vector"
+  )
+})
+
 test_that("infer_type() default method errors for unknown classes", {
   vec <- structure(list(), class = "class_not_supported")