hdf5: fix atread(string)

no need to append null-terminated symbol

modules/hdf/src/hdf5.cpp

@@ -441,10 +441,13 @@ void HDF5Impl::atread(String* value, const String& atlabel)
         CV_Error_(Error::StsInternal, ("Attribute '%s' is not of string type!", atlabel.c_str()));
     }
     size_t size = H5Tget_size(atype);
-    *value = String(size, 0); // allocate space
+    AutoBuffer<char> buf(size);
 
     hid_t atype_mem = H5Tget_native_type(atype, H5T_DIR_ASCEND);
-    H5Aread(attr, atype_mem, const_cast<char*>(value->c_str()));
+    H5Aread(attr, atype_mem, buf.data());
+    if (size > 0 && buf[size - 1] == '\0')
+        size--;
+    value->assign(buf.data(), size);
 
     H5Tclose(atype_mem);
     H5Tclose(atype);

modules/hdf/test/test_hdf5.cpp

@@ -284,9 +284,27 @@ TEST_F(HDF5_Test, test_attribute_String)
 
     m_hdf_io->atwrite(attr_value, attr_name);
 
-    String expected_attr_value;
-    m_hdf_io->atread(&expected_attr_value, attr_name);
-    EXPECT_EQ(attr_value.compare(expected_attr_value), 0);
+    String got_attr_value;
+    m_hdf_io->atread(&got_attr_value, attr_name);
+    EXPECT_EQ(attr_value, got_attr_value);
+
+    m_hdf_io->close();
+}
+
+TEST_F(HDF5_Test, test_attribute_String_empty)
+{
+    reset();
+
+    String attr_name = "test-empty-string";
+    String attr_value;
+
+    m_hdf_io = hdf::open(m_filename);
+
+    m_hdf_io->atwrite(attr_value, attr_name);
+
+    String got_attr_value;
+    m_hdf_io->atread(&got_attr_value, attr_name);
+    EXPECT_EQ(attr_value, got_attr_value);
 
     m_hdf_io->close();
 }