Solve fuzz XMLParser Null-dereference (#5668)

* Refs 22843+22844: Regression test

Signed-off-by: Juanjo Garcia <juanjosegarcia@eprosima.com>

* Refs 22843+22844: Fix

Signed-off-by: Juanjo Garcia <juanjosegarcia@eprosima.com>

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Signed-off-by: Juanjo Garcia <juanjosegarcia@eprosima.com>
(cherry picked from commit dc26c40)

# Conflicts:
#	src/cpp/xmlparser/XMLDynamicParser.cpp
#	test/unittest/xmlparser/XMLParserTests.cpp

Author: juanjo4936