Null pointer crash in excel_read_WINDOW2 on a fuzzed xls file.

Git versions of glib, goffice, gnumeric, libgsf and libxml2.

Test case: http://jutaky.com/fuzzing/gnumeric_case_2303_396369.xls

==19622== ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x7f6a24e2c062 sp 0x7fff4e57bd80 bp 0x7fff4e57bdd0 T0)
AddressSanitizer can not provide additional info.
    #0 0x7f6a24e2c061 in excel_read_WINDOW2 /gnumeric/plugins/excel/ms-excel-read.c:5072
    #1 0x7f6a24e43ee8 in excel_read_sheet /gnumeric/plugins/excel/ms-excel-read.c:6653
    #2 0x7f6a24e4882c in excel_read_BOF /gnumeric/plugins/excel/ms-excel-read.c:6998
    #3 0x7f6a24e49eda in excel_read_workbook /gnumeric/plugins/excel/ms-excel-read.c:7088
    #4 0x7f6a24dbfc8c in excel_enc_file_open /gnumeric/plugins/excel/boot.c:193
    #5 0x7f6a24dc092a in excel_file_open /gnumeric/plugins/excel/boot.c:250
    #6 0x7f6a461b9f1e in go_plugin_loader_module_func_file_open /goffice/goffice/app/go-plugin-loader-module.c:282
    #7 0x7f6a461c2e80 in go_plugin_file_opener_open /goffice/goffice/app/go-plugin-service.c:685 (discriminator 1)
    #8 0x7f6a461cf7cf in go_file_opener_open /goffice/goffice/app/file.c:417
    #9 0x7f6a47332c50 in workbook_view_new_from_input /gnumeric/src/workbook-view.c:1281
    #10 0x7f6a4733343f in workbook_view_new_from_uri /gnumeric/src/workbook-view.c:1341
    #11 0x40a6e0 in main /gnumeric/src/main-application.c:322
    #12 0x7f6a41ab2bc4 in __libc_start_main ??:?
    #13 0x403de8 in _start ??:?
==19622== ABORTING

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Juha Kylmänen
Research Assistant, OUSPG