GMLAS: fix crash when reading CityGML files (fixes r-spatial/sf#2371)

Fixes r-spatial/sf#2371

autotest/ogr/data/gmlas/citygml_empty_lod1.gml

@@ -0,0 +1,3 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<core:CityModel xmlns:bldg="http://www.opengis.net/citygml/building/1.0" xmlns:gml="http://www.opengis.net/gml" xmlns:core="http://www.opengis.net/citygml/1.0" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.opengis.net/citygml/building/1.0 http://repository.gdi-de.org/schemas/adv/citygml/building/1.0/buildingLoD1.xsd http://www.opengis.net/citygml/1.0 http://repository.gdi-de.org/schemas/adv/citygml/1.0/cityGMLBaseLoD1.xsd">
+</core:CityModel>

autotest/ogr/ogr_gmlas.py

@@ -3449,3 +3449,22 @@ def test_ogr_gmlas_get_gml_and_iso_schemas(tmp_path):
         ],
     )
     assert ds
+
+
+###############################################################################
+# Test bugfix for https://github.com/r-spatial/sf/issues/2371
+
+
+@pytest.mark.require_curl()
+def test_ogr_gmlas_bugfix_sf_2371():
+
+    url = (
+        "http://repository.gdi-de.org/schemas/adv/citygml/building/1.0/buildingLoD1.xsd"
+    )
+    conn = gdaltest.gdalurlopen(url, timeout=4)
+    if conn is None:
+        pytest.skip(f"cannot open {url}")
+
+    ds = gdal.OpenEx("GMLAS:data/gmlas/citygml_empty_lod1.gml")
+    lyr = ds.GetLayerByName("address1")
+    assert lyr.GetFeatureCount() == 0

ogr/ogrsf_frmts/gmlas/ogrgmlaslayer.cpp

@@ -1488,8 +1488,8 @@ bool OGRGMLASLayer::InitReader()
 {
     CPLAssert(m_poReader == nullptr);
 
-    m_poReader = m_poDS->CreateReader(m_fpGML);
     m_bLayerDefnFinalized = true;
+    m_poReader = m_poDS->CreateReader(m_fpGML);
     if (m_poReader != nullptr)
     {
         m_poReader->SetLayerOfInterest(this);