widl: If a variable doesn't have a name provide an autogenerated one.

Otherwise widl crashes while generating a typelib for a structure
with an embedded anonymous union:

typedef struct _s
{
    int type;
    union
    {
        int f;
    };
} s;

Signed-off-by: Dmitry Timoshkov <dmitry@baikal.ru>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>

tools/widl/write_msft.c

@@ -1630,6 +1630,9 @@ static HRESULT add_var_desc(msft_typeinfo_t *typeinfo, UINT index, var_t* var)
     unsigned char *namedata;
     int var_num = (typeinfo->typeinfo->cElement >> 16) & 0xffff;
 
+    if (!var->name)
+        var->name = gen_name();
+
     chat("add_var_desc(%d, %s)\n", index, var->name);
 
     id = 0x40000000 + index;