BUG/MINOR: H2: Don't forget to free shared_rx_bufs on failure

BUG/MINOR: H2: Don't forget to free shared_rx_bufs on failure

In h2_init(), if we have a failure while creating the h2c, and we
allocated shared_tx_bufs, don't forget to free it, otherwise we'll have
a memory leak.

This was introduced in 3.1 by commit a891534bfd ("MINOR: mux-h2: allocate
the array of shared rx bufs in the h2c"), so the fix should be backported
as far as 3.2.