[lmi] '-Wconversion-null' warning with latest wx and i686 gcc-8.3

[Greg Chicares]

This is probably benign, but I'll report it anyway.
It occurs with i686 only, not with x86_64.

i686-w64-mingw32-g++ -std=gnu++11 -c -o monodll_msw_listbox.o  
-I/opt/lmi/local/gcc_i686-w64-mingw3
2/wx-ad_hoc/lmi-gcc-8.3-win32/src/tiff/libtiff 
-I/srv/cache_for_lmi/vcs/wxWidgets/src/tiff/libtiff 
-I/srv/cache_for_lmi/vcs/wxWidgets/src/jpeg 
-I/srv/cache_for_lmi/vcs/wxWidgets/src/png -I/srv/cache
_for_lmi/vcs/wxWidgets/src/zlib -I/srv/cache_for_lmi/vcs/wxWidgets/src/regex 
-I/srv/cache_for_lmi/v
cs/wxWidgets/src/expat/expat/lib -D_FILE_OFFSET_BITS=64 
-I/opt/lmi/local/gcc_i686-w64-mingw32/wx-ad
_hoc/lmi-gcc-8.3-win32/lib/wx/include/i686-w64-mingw32-msw-unicode-3.1 
-I/srv/cache_for_lmi/vcs/wxW
idgets/include -I/opt/lmi/local/include  -D__WXMSW__      -DWXBUILDING 
-I/srv/cache_for_lmi/vcs/wxW
idgets/src/stc/scintilla/include 
-I/srv/cache_for_lmi/vcs/wxWidgets/src/stc/scintilla/lexlib -I/srv
/cache_for_lmi/vcs/wxWidgets/src/stc/scintilla/src -D__WX__ -DSCI_LEXER 
-DNO_CXX11_REGEX -DLINK_LEX
ERS -DwxUSE_BASE=1 -DWXMAKINGDLL   -mthreads -Wall -Wundef -Wunused-parameter 
-Wno-ctor-dtor-privac
y -Woverloaded-virtual -O2 -fno-ms-extensions -fno-omit-frame-pointer 
-frounding-math   -I/opt/lmi/
local/include -fno-ms-extensions -fno-omit-frame-pointer -frounding-math 
/srv/cache_for_lmi/vcs/wxW
idgets/src/msw/listbox.cpp

/srv/cache_for_lmi/vcs/wxWidgets/src/msw/listbox.cpp: In member function 
‘virtual bool wxListBox::MSWSetTabStops(const std::vector<int, 
std::allocator<int> >&)’:
/srv/cache_for_lmi/vcs/wxWidgets/src/msw/listbox.cpp:620:69: warning: 
converting to non-pointer type ‘long int’ from NULL [-Wconversion-null]
                        tabStops.empty() ? NULL : (LPARAM)&tabStops[0]) == TRUE;
                                                                     ^