altsvc: reject bad port numbers

altsvc: reject bad port numbers

The existing code tried but did not properly reject alternative services
using negative or too large port numbers.

With this fix, the logic now also flushes the old entries immediately
before adding a new one, making a following header with an illegal entry
not flush the already stored entry.

Report from the ongoing source code audit by Trail of Bits.

Adjusted test 356 to verify.

Closes #9607