[PATCH][widl] Fix uninitialized read in write_union_tfs

[Raphael Isemann]

The summary of how the bug happens is:

1. union_memsize gets called with a pointer to uninitialized `dummy`.
```
static unsigned int write_union_tfs(FILE *file, const attr_list_t *attrs,
                                   type_t *type, unsigned int *tfsoff)
{
   [...]
   unsigned int dummy;
   [...]
   size = union_memsize(fields, &dummy);
```

2. `union_memsize` reads `pmaxa` (which points to dummy) to set the
initial value of align.

```
static unsigned int union_memsize(const var_list_t *fields, unsigned
int *pmaxa) {
   unsigned int size, maxs = 0;
   unsigned int align = *pmaxa;
   const var_t *v;

   if (fields) LIST_FOR_EACH_ENTRY( v, fields, const var_t, entry )
   {
       /* we could have an empty default field with NULL type */
       if (v->declspec.type)
       {
           size = type_memsize_and_alignment(v->declspec.type, &align);
           if (maxs < size) maxs = size;
           if (*pmaxa < align) *pmaxa = align;
       }
   }
```

`type_memsize_and_alignment` then descends down and passes along the
uninitialized `align`. `type_memsize_and_alignment` will use this to find
the required alignment for the type (and will round up the computed type size
to the uninitialized alignment if necessary).

This patch just initializes `dummy` to 0 (which means the 'real'
alignment value) should set the value to a real alignment.
---
tools/widl/typegen.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/widl/typegen.c b/tools/widl/typegen.c
index b2d9ab5..eb0fa5e 100644
--- a/tools/widl/typegen.c
+++ b/tools/widl/typegen.c
@@ -3327,7 +3327,7 @@ static unsigned int write_union_tfs(FILE *file,
const attr_list_t *attrs,
    unsigned int nbranch = 0;
    type_t *deftype = NULL;
    short nodeftype = 0xffff;
-    unsigned int dummy;
+    unsigned int dummy = 0;
    var_t *f;

    if (processed(type) &&
--
2.35.1