dhcp: check length of option before accessing

Prevent Rust index out of bounds panic.

Redmine issue:
https://redmine.openinfosecfoundation.org/issues/2571

rust/src/dhcp/logger.rs

@@ -190,6 +190,7 @@ impl DHCPLogger {
     }
 
     fn log_opt_type(&self, js: &Json, option: &DHCPOptGeneric) {
+        if option.data.len() > 0 {
             let dhcp_type = match option.data[0] {
                 DHCP_TYPE_DISCOVER => "discover",
                 DHCP_TYPE_OFFER => "offer",
@@ -203,6 +204,7 @@ impl DHCPLogger {
             };
             js.set_string("dhcp_type", dhcp_type);
         }
+    }
 
     fn log_opt_parameters(&self, js: &Json, option: &DHCPOptGeneric) {
         let params = Json::array();