Fix some error conditions in HDF5 backend

Author: franzpoeschel

src/IO/HDF5/HDF5IOHandler.cpp

@@ -751,7 +751,7 @@ void HDF5IOHandlerImpl::openPath(
 
     node_id = H5Gopen(
         file.id, concrete_h5_file_position(writable->parent).c_str(), gapl);
-    if (node_id <= 0)
+    if (node_id < 0)
     {
         throw error::ReadError(
             error::AffectedObject::Group,
@@ -770,7 +770,7 @@ void HDF5IOHandlerImpl::openPath(
         if (!auxiliary::ends_with(path, '/'))
             path += '/';
         path_id = H5Gopen(node_id, path.c_str(), gapl);
-        if (path_id <= 0)
+        if (path_id < 0)
         {
             throw error::ReadError(
                 error::AffectedObject::Group,
@@ -1712,7 +1712,7 @@ void HDF5IOHandlerImpl::readAttribute(
     std::vector<hsize_t> maxdims(ndims, 0);
 
     status = H5Sget_simple_extent_dims(attr_space, dims.data(), maxdims.data());
-    if (obj_id < 0)
+    if (status != ndims)
     {
         throw error::ReadError(
             error::AffectedObject::Attribute,