Benign unsigned integer overflow in Parcel

The realloc case in continueWrite did not update the
gParcelGlobalAllocCount value when an allocation occurred.

In addition, there are conditions that could cause the
gParcelGlobalAllocCount value to be decremented below 0, resulting
in a benign unsigned integer overflow that can cause corrupted values
to be returned through system profiling mechanisms.

BUG: 23972600
Change-Id: Ibe07db91a811a04b486760eb78d81c926ba8503d