InsetIndex: add a missing destructor for IndexNode and call it from xhtml() to avoid...

InsetIndex: add a missing destructor for IndexNode and call it from xhtml() to avoid leaking resources.

Corresponding Coverity finding:

** CID 382778:  Resource leaks  (RESOURCE_LEAK)
/home/lasgoutt/src/lyx/coverity/lyx/src/insets/InsetIndex.cpp: 1909 in _ZNK3lyx15InsetPrintIndex5xhtmlB5cxx11ERNS_9XMLStreamERKNS_12OutputParamsE()

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*** CID 382778:  Resource leaks  (RESOURCE_LEAK)
/home/lasgoutt/src/lyx/coverity/lyx/src/insets/InsetIndex.cpp: 1909 in _ZNK3lyx15InsetPrintIndex5xhtmlB5cxx11ERNS_9XMLStreamERKNS_12OutputParamsE()
1903            }
1904
1905            xs << xml::EndTag("ul");
1906            xs << xml::CR();
1907            xs << xml::EndTag("div");
1908
>>>     CID 382778:  Resource leaks  (RESOURCE_LEAK)
>>>     Variable "index_root" going out of scope leaks the storage it points to.
1909            return ods.str();
1910     }
1911