Add assert() statements and eliminate needless variable assignments in order

to get the clang scan-build utility to report zero problems against the
SQLite core.  Clang's static analysis did find one real problem - but it was
in the command-line shell, not in the SQLite core.

FossilOrigin-Name: 60fee9574b0125705787e33c16f116cf188c8323

src/main.c

@@ -2282,6 +2282,7 @@ opendb_out:
     sqlite3_mutex_leave(db->mutex);
   }
   rc = sqlite3_errcode(db);
+  assert( db!=0 || rc==SQLITE_NOMEM );
   if( rc==SQLITE_NOMEM ){
     sqlite3_close(db);
     db = 0;