Fasttable: Fix oneof submessage merging corruption.

When a submessage field within a oneof is encountered, the fast path
parser was incorrectly reusing any existing message object at that
offset even if the oneof case had changed. We now correctly zero out
the pointer if the oneof case has changed, forcing the allocation of
a new submessage object of the correct type.

PiperOrigin-RevId: 917836422