Null pointer segfault on an invalid UTF8 character on html2tex conversion.

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

Test case: http://jutaky.com/fuzzing/gnumeric_case_19784_236614.2tex.min.html

"ssconvert gnumeric_case_19784_236614.2tex.min.html out.tex":

==23416== ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x7f4e2db1595a sp 0x7fffc2fdc7c0 bp 0x7fffc2fdc7f0 T0)
AddressSanitizer can not provide additional info.
    #0 0x7f4e2db15959 in latex_fputs_latin /gnumeric/plugins/html/latex.c:330 (discriminator 1)
    #1 0x7f4e2db15b7c in latex_fputs /gnumeric/plugins/html/latex.c:415
    #2 0x7f4e2db1a60f in latex2e_table_write_cell /gnumeric/plugins/html/latex.c:1528
    #3 0x7f4e2db1a850 in latex_table_file_save /gnumeric/plugins/html/latex.c:1600
    #4 0x7f4e4da0c0f7 in go_plugin_loader_module_func_file_save /goffice/goffice/app/go-plugin-loader-module.c:366
    #5 0x7f4e4da10e6b in go_plugin_file_saver_save /goffice/goffice/app/go-plugin-service.c:948 (discriminator 1)
    #6 0x7f4e4da16e33 in go_file_saver_save /goffice/goffice/app/file.c:848
    #7 0x7f4e4e3862ad in wbv_save_to_output /gnumeric/src/workbook-view.c:1055
    #8 0x7f4e4e38650f in wb_view_save_to_uri /gnumeric/src/workbook-view.c:1092
    #9 0x7f4e4e38681d in wb_view_save_as /gnumeric/src/workbook-view.c:1128
    #10 0x406473 in convert /gnumeric/src/ssconvert.c:788
    #11 0x406893 in main /gnumeric/src/ssconvert.c:860
    #12 0x7f4e495dca14 in __libc_start_main ??:?
    #13 0x403708 in _start ??:?
==23416== ABORTING

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Juha Kylmänen
Research Assistant, OUSPG