remove_warning in db.c

just remove some compile warning in CentOS 5.x(gcc version 4.1.2 20080704)

before
```c
db.c: In function ‘scanGenericCommand’:
db.c:419: warning: ‘patlen’ may be used uninitialized in this function
db.c:418: warning: ‘pat’ may be used uninitialized in this function
```