9.2.x: INKVConnInternal::do_io_*: handle null buffer

[bneradt]

It's common for users of VC's to cancel io via a 0 byte, nullptr read or
write on the VC. INKVConnInternal::do_io_read and
INKVConnInternal::do_io_write were not prepared to handle such
cancellations. This updates them to handle this gracefully rather than
crashing on a nullptr dereference. This change is was found to be needed
for the multiplexer plugin for handling HttpTunnel aborts.

For reference, see, for example, UnixNetVConnection::do_io_read which
handles a nullptr MIOBuffer. This basically copies that logic into
INKVConnInternal so it handles cancellation gracefully.

(cherry picked from commit 7afd9e3)

[Copilot]

Pull request overview

Updates the internal SDK VConnection implementation (INKVConnInternal) to safely handle common I/O cancellation patterns where plugins call do_io_read / do_io_write with nbytes == 0 and a null buffer/reader. This aligns behavior more closely with established NetVC handling (e.g., UnixNetVConnection) by avoiding null dereferences during cancellation flows (notably for HttpTunnel abort handling in the multiplexer plugin).

Changes:

• Guard INKVConnInternal::do_io_read against null MIOBuffer* by clearing the VIO buffer accessor instead of dereferencing.
• Guard INKVConnInternal::do_io_write against null IOBufferReader* by clearing the VIO buffer accessor instead of dereferencing.
• Only schedule an immediate event when a non-null buffer/reader is provided (and, for writes, when there is data available).

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