external/cairo: Avoid UBSan invalid-null-argument

...as seen during CppunitTest_sw_uiwriter4:

> [_RUN_____] SwUiWriterTest4::testEmojiAutoCorrect
cairo-surface.c:2852:28: runtime error: null pointer passed as argument 2, which is declared to never be null
> /usr/include/string.h:44:28: note: nonnull attribute specified here
>  #0 in _cairo_surface_show_text_glyphs at workdir/UnpackedTarball/cairo/src/cairo-surface.c:2852:9 (instdir/program/libcairo.so.2 +0x9ac09e)
>  #1 in _cairo_gstate_show_text_glyphs at workdir/UnpackedTarball/cairo/src/cairo-gstate.c:2077:15 (instdir/program/libcairo.so.2 +0x65a4af)
>  #2 in _cairo_default_context_glyphs at workdir/UnpackedTarball/cairo/src/cairo-default-context.c:1315:12 (instdir/program/libcairo.so.2 +0x62404f)
>  #3 in cairo_show_glyphs at workdir/UnpackedTarball/cairo/src/cairo.c:3629:14 (instdir/program/libcairo.so.2 +0xa6c77f)
>  #4 in CairoTextRender::DrawTextLayout(GenericSalLayout const&, SalGraphics const&) at vcl/unx/generic/gdi/cairotextrender.cxx:265:9 (instdir/program/libvcllo.so +0xae46aa3)

Change-Id: Ifa22046bb35a872c4db38130a7ae4c9b758ccbc6
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/121473
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

0 files changed, 0 insertions, 0 deletions