Out-of-bounds read on gnumeric2xls conversion.

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

Test case: http://jutaky.com/fuzzing/gnumeric_case_32445_24838.2xls.gnumeric

==28934== ERROR: AddressSanitizer: SEGV on unknown address 0x600400230000 (pc 0x7f1a342b6870 sp 0x7fff089740b0 bp 0x7fff089740d0 T0)
AddressSanitizer can not provide additional info.
    #0 0x7f1a342b686f in g_utf8_pointer_to_offset /glib/glib/gutf8.c:402
    #1 0x7f1a16499027 in excel_write_RSTRING /gnumeric/plugins/excel/ms-excel-write.c:3465 (discriminator 2)
    #2 0x7f1a164999da in excel_write_cell /gnumeric/plugins/excel/ms-excel-write.c:3501
    #3 0x7f1a164b17b6 in excel_sheet_write_block /gnumeric/plugins/excel/ms-excel-write.c:5235
    #4 0x7f1a164b46f1 in excel_write_sheet /gnumeric/plugins/excel/ms-excel-write.c:5417 (discriminator 2)
    #5 0x7f1a164c2bfc in excel_write_workbook /gnumeric/plugins/excel/ms-excel-write.c:6269 (discriminator 2)
    #6 0x7f1a164c39b8 in excel_write_v8 /gnumeric/plugins/excel/ms-excel-write.c:6322
    #7 0x7f1a163dfeca in excel_save /gnumeric/plugins/excel/boot.c:281
    #8 0x7f1a163e0721 in excel_biff8_file_save /gnumeric/plugins/excel/boot.c:322
    #9 0x7f1a3808d3bc in go_plugin_loader_module_func_file_save /goffice/goffice/app/go-plugin-loader-module.c:366
    #10 0x7f1a380980a4 in go_plugin_file_saver_save /goffice/goffice/app/go-plugin-service.c:948 (discriminator 1)
    #11 0x7f1a380a65e0 in go_file_saver_save /goffice/goffice/app/file.c:848
    #12 0x7f1a391f4440 in wbv_save_to_output /gnumeric/src/workbook-view.c:1059
    #13 0x7f1a391f4c76 in wb_view_save_to_uri /gnumeric/src/workbook-view.c:1096
    #14 0x7f1a391f55fd in wb_view_save_as /gnumeric/src/workbook-view.c:1132
    #15 0x40a9ab in convert /gnumeric/src/ssconvert.c:788
    #16 0x40b6b2 in main /gnumeric/src/ssconvert.c:860
    #17 0x7f1a3397fbc4 in __libc_start_main ??:?
    #18 0x403f68 in _start ??:?
==28934== ABORTING

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Juha Kylmänen
Research Assistant, OUSPG