Fix zero init of class members in message handler

There are instances of class members not being zero initialized in the
message handler. To avoid possible issues with using the class members
when they have an intermediate value, the constructors have been updated
to zero initialize all the member variables.

Change-Id: Ib539410a8f1486f8155272731f5cb7006c22ac7a
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>