socket: reject over-long proxy response line

socket: reject over-long proxy response line

fixes a one byte stack overflow when using RSYNC_PROXY with a
malicious proxy.

Reach: only when RSYNC_PROXY is set and a malicious or MITM'd
proxy returns the pathological response.  The byte written is
always '\0' and the attacker doesn't choose the offset, so impact
is corruption of one adjacent stack byte and possible later
misbehaviour or crash -- no information disclosure beyond the
existing rprintf of buffer contents.

Reported by Aisle Research via Michal Ruprich