MySQL statement can not be identified

Hi,

I found the existence of such MYSQL statement can not be identified successfully.

the test case:
```
require 'libinjection'

sqli = "DELETE t1, t2 FROM t1 INNER JOIN t2 INNER JOIN t3 WHERE t1.id=t2.id AND t2.id=t3.id;"

sql_state = libinjection.sqli_state()
libinjection.sqli_init(sql_state, sqli, sqli:len(), 0)

print(libinjection.is_sqli(sql_state))
```
it‘s will print:
```
0
```