inpcb: Restore some NULL checks of credential pointers

At least one out-of-tree port (net-mgmt/ng_ipacct) depends on being able
to call in_pcblookup_local() with cred == NULL, so the MFC of commit
ac1750dd143e ("inpcb: Remove NULL checks of credential references")
broke compatibility.

Restore a subset of the NULL checks to avoid breaking the module in the
13.3 release. This is a direct commit to stable/13.

PR: 276868