BUG/MEDIUM: applet: check buffer sizes before HTX transfer

BUG/MEDIUM: applet: check buffer sizes before HTX transfer

The appctx_htx_snd_buf() fast path uses b_xfer() when the destination
HTX message is empty and the source contains a complete HTX message.
However, it does not verify that both buffers have the same capacity.

When tune.bufsize.large is configured and wait-for-body uses
use-large-buffer, the channel buffer may be larger than the applet
input buffer. The fast path can then attempt to transfer more data
than the destination can hold, triggering a fatal bounds check and
terminating the worker process.

Only use the fast path when the source and destination buffers have
the same capacity. Otherwise, use the existing HTX block-wise transfer
path.

[cf: Must be backported to 3.4]