Fix use-of-uninitialized-value in XML parser

[memurro000]

Fix use-of-uninitialized-value in XML parser

Problem: MemorySanitizer detects use-of-uninitialized-value at line 96 in xml_parser_read_rapidxml.hpp when parsing malformed XML input through GraphML parser (issue #131).

Root cause: The XML parser attempts to read from streams that are in bad state or contain uninitialized data, which leads to undefined behavior in RapidXML.

Solution: Add early stream validation in read_xml_internal():

• Check stream state with stream.good() before any operations
• Verify non-empty input with stream.peek() != eof()
• Throw appropriate xml_parser_error for invalid inputs

Testing: The fix resolves the MemorySanitizer warning while maintaining backward compatibility. Empty streams and bad stream states now throw clear exceptions instead of causing undefined behavior.

Files changed:

• libs/property_tree submodule updated with the security fix

Fixes: #131

[pdimov]

I suspect that the actual issue here is #129.