[systemd-sonic-generator] replace sscanf with strtol (sonic-net#15357)

#### Why I did it
Avoid 'sscanf()' for number conversions. Its use can lead to undefined behavior, slow processing, and integer overflows. Instead prefer the 'strto*()' family of functions.

#### How I did it
replace sscanf with strtol
#### How to verify it
Manual test

src/systemd-sonic-generator/systemd-sonic-generator.c

@@ -569,7 +569,7 @@ int get_num_of_asic() {
                     str_num_asic = strtok_r(NULL, "=", &saveptr);
                     strip_trailing_newline(str_num_asic);
                     if (str_num_asic != NULL){
-                        sscanf(str_num_asic, "%d",&num_asic);
+                        num_asic = strtol(str_num_asic, NULL, 10);
                     }
                     break;
                 }