CLEANUP: mux-h2: clarify that bptr is not NULL when calling to h2_get_buf()

CLEANUP: mux-h2: clarify that bptr is not NULL when calling to h2_get_buf()

In h2_frt_transfer_data(), we first check h2s_rxbuf_tail() and if it's
NULL, we get out. A bit later we pass that function's return to
h2_get_buf(), which dereferences it. It's OK since it was checked. But
depending on optimization levels and tiny extra fixes, gcc-9.5 (at least)
sometimes thinks it sees a possible null-deref and emits a warning. Let's
just use the known safe version of h2s_rxbuf_tail() since we know for sure
that the pointer is valid so that the compiler is no longer confused. A
comment was added at the top of the function to clarify that the pointer
may not be NULL.