Fixed a memory bug with RepeatedField#+.

[haberman]

We need to fuse the arrays so that the second array's data stays live.

Fixes: #8420

I was only able to reproduce the failure under Valgrind. Our Ruby library doesn't have Valgrind tests at the moment, so I didn't add a regression test for now. When we are able to add Valgrind tests, the following snippet triggered the bug:

require 'google/protobuf'

GC.stress = true
x = Google::Protobuf::RepeatedField.new(:string, [])
y = Google::Protobuf::RepeatedField.new(:string, ["1", "2", "3"])
z = x + y
y = nil
puts z[0]