Add NULL checks on all sqlite3_vfs_find(0) calls. This is not strictly

necessary.  There are no vulnerabilities here.  However, adding these
checks avoids unnecessary static analyzer complaints.
[forum:/forumpost/ce1193be15|Forum post ce1193be15].

FossilOrigin-Name: 272a15b9f418fb0b31a9808f7c42c20cf52318035ff98935d8e8519634357e8d