streaming/buffer: improve integer handling safety

streaming/buffer: improve integer handling safety

Unsafe handling of buffer offset and to be inserted data's length
could lead to a integer overflow. This in turn would skip growing
the target buffer, which then would be memcpy'd into, leading to
an out of bounds write.

This issue shouldn't be reachable through any of the consumers of
the API, but to be sure some debug validation checks have been
added.

Bug: #6903.