fix(r/adbcdrivermanager): Ensure that class of object is checked befo…

…re calling R_ExternalPtrAddrFn (#1989)

In the development version of R, calling `R_ExternalPtrAddrFn()` on an
SEXP that is not an external pointer will raise an error. Because we
test the error message, the order of our check vs. the call to
`R_ExternalPtrAddrFn()` matters. In general, it makes more sense to do
our own check first anyway!


https://cran.r-project.org/web/checks/check_results_adbcdrivermanager.html

```
   > test_check("adbcdrivermanager")
    [ FAIL 1 | WARN 0 | SKIP 3 | PASS 176 ]
    
    ══ Skipped tests (3) ═══════════════════════════════════════════════════════════
    • On CRAN (3): 'test-driver_log.R:19:3', 'test-helpers.R:22:3',
      'test-helpers.R:112:3'
    
    ══ Failed tests ════════════════════════════════════════════════════════════════
    ── Error ('test-radbc.R:188:3'): invalid external pointer inputs generate errors ──
    Error in `adbc_database_init_default(driver, list(...))`: R_ExternalPtrAddrFn: argument of type STRSXP is not an external pointer
```

r/adbcdrivermanager/src/radbc.cc

@@ -112,12 +112,13 @@ extern "C" SEXP RAdbcLoadDriver(SEXP driver_name_sexp, SEXP entrypoint_sexp) {
 }
 
 extern "C" SEXP RAdbcLoadDriverFromInitFunc(SEXP driver_init_func_xptr) {
-  auto driver_init_func =
-      reinterpret_cast<AdbcDriverInitFunc>(R_ExternalPtrAddrFn(driver_init_func_xptr));
   if (!Rf_inherits(driver_init_func_xptr, "adbc_driver_init_func")) {
     Rf_error("Expected external pointer with class '%s'", "adbc_driver_init_func");
   }
 
+  auto driver_init_func =
+      reinterpret_cast<AdbcDriverInitFunc>(R_ExternalPtrAddrFn(driver_init_func_xptr));
+
   SEXP driver_xptr = PROTECT(adbc_allocate_xptr<AdbcDriver>());
   R_RegisterCFinalizer(driver_xptr, &finalize_driver_xptr);
   auto driver = adbc_from_xptr<AdbcDriver>(driver_xptr);
@@ -148,12 +149,13 @@ extern "C" SEXP RAdbcDatabaseNew(SEXP driver_init_func_xptr) {
   adbc_error_stop(status, &error);
 
   if (driver_init_func_xptr != R_NilValue) {
-    auto driver_init_func =
-        reinterpret_cast<AdbcDriverInitFunc>(R_ExternalPtrAddrFn(driver_init_func_xptr));
     if (!Rf_inherits(driver_init_func_xptr, "adbc_driver_init_func")) {
       Rf_error("Expected external pointer with class '%s'", "adbc_driver_init_func");
     }
 
+    auto driver_init_func =
+        reinterpret_cast<AdbcDriverInitFunc>(R_ExternalPtrAddrFn(driver_init_func_xptr));
+
     status = AdbcDriverManagerDatabaseSetInitFunc(database, driver_init_func, &error);
     adbc_error_stop(status, &error);
   }