move symbol lookup of sumof expr to the parser

replace the complicated symboltable lookup for sumof expr
by accessing the lenfield of the expr-object.

This requires the corresponding patch for xcb/proto
which sets the lenfield accordingly.

This should be OK because for official releases we define
that dependency in the build system.

For getting versions off the HEAD of the git repo, it should
be obvious that xcb/proto and xcb/libxcb have to be updated together.

I have tested this patch and it generates exactly the same code
as before.

Tested-by: Christian Linhart <chris@demorecorder.com>
Signed-off-by: Christian Linhart <chris@demorecorder.com>

src/c_client.py

@@ -1661,14 +1661,7 @@ def _c_accessor_get_expr(expr, field_mapping):
         return c_name
     elif expr.op == 'sumof':
         # locate the referenced list object
-        field = None
-        for f in expr.lenfield_parent.fields:
-            if f.field_name == expr.lenfield_name:
-                field = f
-                break
-
-        if field is None:
-            raise Exception("list field '%s' referenced by sumof not found" % expr.lenfield_name)
+        field = expr.lenfield
         list_name = field_mapping[field.c_field_name][0]
         c_length_func = "%s(%s)" % (field.c_length_name, list_name)
         c_length_func = _c_accessor_get_expr(field.type.expr, field_mapping)