base: Fix unitialized storage

base: Fix unitialized storage

The bitunion is not being initialized on constructor to avoid
performance overhead, and that generated a maybe-unitialized
error when a sub-class was being copied before assigned in
serialize's parseParam() in some compilers.

This patch adds zero-initialization to the problematic variable
to appease the compiler.

Change-Id: I90fa6aa356b3e14ec25e3294b17ed10f429a9a38
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/15635
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>