MINOR: h3: remove dead code on h3_rcv_buf() error path

MINOR: h3: remove dead code on h3_rcv_buf() error path

Previous patches rework error handling and glitch reporting in
h3_rcv_buf() after a connection level error occured. A similar pattern
is adopted after each frame parsers : when a connection level error is
encountered, glitch is incremented and parsing is immediately
interrupted using early return via err label.

To ensure the code remains safe though with future evolution, h3c <err>
check on h3_rcv_buf() epilogue is replaced so that qcc_set_error() is
only set if not already done, which is currently unnecessary. A
WARN_ON() should help to detect such cases.