CLEANUP: qpack: simplify length checks in qpack_decode_fs()

CLEANUP: qpack: simplify length checks in qpack_decode_fs()

This patch simplifies the decoding loop by merging the variable-length
integer truncation check (len == -1) with the subsequent buffer
availability check (len < length).

This removes redundant code blocks and improves readability without
changing the decoding logic.

Note that the second removal is correct, as the check was duplicate and
unnecessary."