toolbox: fix memory leak

free img

Test: make WITH_TIDY=1 mmma system/core/toolbox
Change-Id: I2f8feff0eddca763ee4e7fc9867480af60c8a911

toolbox/newfs_msdos.c

@@ -741,6 +741,7 @@ int newfs_msdos_main(int argc, char *argv[])
                 exit(1);
             }
         }
+        free(img);
     }
     return 0;
 }