glsl2: Use talloc_zero_size instead of talloc_size to allocate ast_node objects.

This is a zero-ing function, (like calloc), to avoid bugs due to
accessing uninitialized values. Thanks to valgrind for noticing the
use of uninitialized values.

src/glsl/ast.h

@@ -42,7 +42,7 @@ public:
    {
       void *node;
 
-      node = talloc_size(ctx, size);
+      node = talloc_zero_size(ctx, size);
       assert(node != NULL);
 
       return node;