BUG/MINOR: proxy: Fix input data copy when an error is captured

BUG/MINOR: proxy: Fix input data copy when an error is captured

In proxy_capture_error(), input data are copied in the error snapshot. The copy
must take care of the data wrapping. But the length of the first block is
wrong. It should be the amount of contiguous input data that can be copied
starting from the input's beginning. But the mininum between the input length
and the buffer size minus the input length is used instead. So it is a problem
if input data are wrapping or if more than the half of the buffer is used by
input data.

This patch must be backported as far as 1.9.