pUnoReturn should be null when UNO methode is VOID

the problem is that at least the msvc_win32_x86-64 bridge's 
unoInterfaceProxyDispatch 
(bridges/source/cpp_uno/msvc_win32_x86-64/uno2cpp.cxx) 
requires pUnoReturn to be a nullptr when the UNO method has VOID 
return type (see computation of retKind in cpp_call in the same file), 
but that IdlInterfaceMethodImpl::invoke doesn't set up the arguments 
according to that expectation.


Change-Id: I187a997300571cd9822de2eeacf7ad887ad00a4f
Reviewed-on: https://gerrit.libreoffice.org/69495Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>

stoc/source/corereflection/criface.cxx

@@ -579,7 +579,7 @@ Any SAL_CALL IdlInterfaceMethodImpl::invoke( const Any & rObj, Sequence< Any > &
         // end of a "short" struct by writing the full contents of a "long"
         // register); so create enough space here (assuming that no ABI requires
         // padding larger than 16 byte boundaries):
-        void * pUnoReturn = alloca( multipleOf16(pReturnType->nSize) );
+        void * pUnoReturn = (pReturnType->nSize == 0) ? nullptr : alloca( multipleOf16(pReturnType->nSize) );
         void ** ppUnoArgs = static_cast<void **>(alloca( sizeof(void *) * nParams *2 ));
         typelib_TypeDescription ** ppParamTypes = reinterpret_cast<typelib_TypeDescription **>(ppUnoArgs + nParams);