bcachefs: fix possible null-ptr-deref in __bch2_ec_stripe_head_get()

bcachefs: fix possible null-ptr-deref in __bch2_ec_stripe_head_get()

The function ec_new_stripe_head_alloc() returns nullptr if kzalloc()
fails. It is crucial to verify its return value before dereferencing
it to avoid a potential nullptr dereference.

Fixes: 035d72f72c91 ("bcachefs: bch2_ec_stripe_head_get() now checks for change in rw devices")
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>