fix implicit zero initializer.

fix implicit zero initializer.

The C standard requires that, when initializing an aggregate, all
fieds not explicitly initialized shall be implicity zero-initialized
(more exactly "the same as objects that have static storage duration"
[6.7.9.21]).

Until now sparse didn't did this.
Fix this (when an initializer is present and the object not a scalar)
by first storing zeroes in the whole object before doing the
initialization of each fields explicitly initialized.

Note 1: this patch initialize the *whole* aggregate while the
	standard only requires that existing fields are initialized.
	Thanks to Linus to notice this.
Note 2: this implicit initialization is not needed if all fields are
        explicitly initialized but is done anyway, for the moment.
Note 3: the code simplify nicely when there is a single field that is
        initialized, much less so when there is several ones.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>