BUG/MINOR: shctx: fix shctx_row_data_get() when offset exceeds a block

BUG/MINOR: shctx: fix shctx_row_data_get() when offset exceeds a block

The loop advanced the block pointer only after the copy, but skipped
blocks preceding <offset> with a continue, which bypassed that advance.
As a result any read starting past the first block copied nothing.

This notably affects conditional requests: should_send_notmodified_response()
reads the stored ETag at entry->etag_offset, so a cached response whose
ETag header sits beyond the first block fails revalidation and is served
in full (200) instead of 304.

Rewrite the loop as a for() so the block pointer always advances,
regardless of whether the current block is copied or skipped.

This should be backported as far as 2.6.