vppinfra: fix longstanding corner case bug in serialize_get()

serialize_get() -> serialize_write_not_inline(...) was losing track of
the current buffer index when it managed to empty the overflow vector
but had to turn around and use it again.

Test-case added to test_serialize.c.

This issue dates from 2010.

Type: fix

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I024a03f7a50fd6df543ddbc7c45d85def4f1981d