BUG/MINOR: mux-h2: Use the dummy error when decoding headers for a closed stream

BUG/MINOR: mux-h2: Use the dummy error when decoding headers for a closed stream

Since the commit 6884aa3e ("BUG/MAJOR: mux-h2: Handle HEADERS frames received
after a RST_STREAM frame"), HEADERS frames received for an unknown or already
closed stream are decoded. Once decoded, an error is reported for the
stream. But because it is a dummy stream (h2_closed_stream), its state cannot be
changed. So instead, we must return the dummy error stream (h2_error_stream).

This patch must be backported to 2.0 and 1.9.