proxy_protocol: fix off-by-one stack buffer overflow in PROXY v1 head…

[uwezkhan]

The header parsing loop could append a null terminator past the end of
the buffer when processing maximum-length PROXY v1 headers.

Fix by increasing the buffer size by one byte and using bounded reads
while preserving full header parsing.

Treat zero-length reads as connection close in this context.

Add a regression test for maximum-length PROXY headers.