Ensure that Vector::erase() returns a valid iterator

Vector::erase may reallocate the Vector's storage while removing an element.
However, erase() calls begin() before calling removeItemsAt(), thus caching
a pointer the the Vector's old storage.  If the storage is reallocated,
the iterator returned by erase() will be based on the old storage pointer
and will thus be invalid.

Change-Id: I2450c55fd418e6b1c558a4ca7c024573abbaa098